# He must go to school yesterday.



## grammar-in-use

Hello everyone,

It'd be nonsensical to say "He *must* go to school *yesterday*", right? Then what about "He *told* me that he *must* go to school *yesterday*"?
The latter seems to be acceptable, right? So, too, is "He *had* to go to school yesterday".

Could some native speakers shed some light on the three sentences above? I'd really appreciate it.


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## tunaafi

'Must' as no past tense. We normally use 'had to' instead. However, in reported speech, it is possible to use 'must' when we would normally backshift. I don't know why.


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## grammar-in-use

Thank you for your quick reply.
So all we can say is, that's just the way it is, right?
There should be no problem with "He *must* go to school *today*/*tomorrow*", is there?


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## tunaafi

None at all.


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## Logos_

grammar-in-use said:


> There should be no problem with "He *must* go to school *today*/*tomorrow*", is there?



Those are acceptable.


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## tunaafi

grammar-in-use said:


> So all we can say is, that's just the way it is, right?


That's all* I *can say. Others may know more about this.


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## handsomechuck

tunaafi said:


> 'Must' as no past tense. We normally use 'had to' instead. However, in reported speech, it is possible to use 'must' when we would normally backshift. I don't know why.



Must can be used in the past. Sometimes, especially in (older) writing, where we would use "had to," you will see something like "The building was on fire. He must escape or perish."


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## grammar-in-use

Logos_ said:


> Those are acceptable.


Thank you!
So, "He *must* go to school *yesterday*" does not sound that right to you natives, whereas "He *told* me that he *must* go to school *yesterday*" is perfectly acceptable, right? I'm just wondering why...


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## tunaafi

handsomechuck said:


> Must can be used in the past. Sometimes, especially in (older) writing, where we would use "had to," you will see something like "The building was on fire. He must escape or perish."


I'd be inclined to regard that as an example of_ free indirect speech_.


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## Logos_

A more natural way to say it would be "he told me he should have gone to school yesterday" or "he told me he had to go...etc." "He told me he must go yesterday" strikes me as very strange.


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## grammar-in-use

handsomechuck said:


> Must can be used in the past. Sometimes, especially in (older) writing, where we would use "had to," you will see something like "The building was on fire. He must escape or perish."


Thank you. 
But still, "He *must* go to school *yesterday*" doesn't sound right to you guys, does it?


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## grammar-in-use

tunaafi said:


> I'd be inclined to regard that as an example of_ free indirect speech_.





handsomechuck said:


> Must can be used in the past. Sometimes, especially in (older) writing, where we would use "had to," you will see something like "The building was on fire. He must escape or perish."



On second thoughts, the "must" in "He *must* escape or perish" has a different sense from that in "He *must* go to school", right?


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## Logos_

grammar-in-use said:


> Thank you.
> But still, "He *must* go to school *yesterday*" doesn't sound right to you guys, does it?



No, I would rephrase using "should have gone" or "had to."


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## handsomechuck

grammar-in-use said:


> Thank you.
> But still, "He *must* go to school *yesterday*" doesn't sound right to you guys, does it?



Agree. It can't be used that way.


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## grammar-in-use

Logos_ said:


> A more natural way to say it would be "he told me he should have gone to school yesterday" or "he told me he had to go...etc." "He told me he must go yesterday" strikes me as very strange.


But "he told me he should have gone to school yesterday" is different in meaning from "he told me he had to/must go...", right?


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## Logos_

grammar-in-use said:


> But "he told me he should have gone to school yesterday" is different in meaning from "he told me he had to/must go...", right?


"Had to" is stronger (closer to must/was required to go) than "should have" and I would choose between them depending on what you want to express.


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## lingobingo

grammar-in-use said:


> …"He *told* me that he *must* go to school *yesterday*" is perfectly acceptable, right? I'm just wondering why...


Has anyone said that’s acceptable? It’s not to me!

He told me yesterday that he had to go to school  ​He told me that he had to go to school yesterday ​
But:

He told me yesterday that he must go to school (i.e. attend on a regular basis like everyone else) or he will be excluded permanently ​
But even this meaning doesn’t work with “yesterday” at the end of the sentence.


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## grammar-in-use

Logos_ said:


> "Had to" is stronger (closer to must/was required to go) than "should have" and I would choose between them depending on what you want to express.


OK, as I understand it, "he told me he should have gone to school yesterday", but the fact is that he didn't go to school yesterday.
I would understand "he told me he had to go to school yesterday" as "so he did go..."


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## Logos_

grammar-in-use said:


> OK, as I understand it, "he told me he should have gone to school yesterday", but the fact is that he didn't go to school yesterday.
> I would understand "he told me he had to go to school yesterday" as "so he did go..."



It would all depend on what you wanted to express and then we could make a construction to suit that situation exactly: Did he go? Was he required to go? Should he have gone? Did he not go despite being required?


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## grammar-in-use

lingobingo said:


> Has anyone said that’s acceptable? It’s not to me!


Please can you refer to post #2, which I'd understand it as acceptable (if I understand correctly).


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## grammar-in-use

Logos_ said:


> It would all depend on what you wanted to express and then we could make a construction to suit that situation exactly: Did he go? Was he required to go? Should he have gone? Did he not go despite being required?


I wanted to convey "he did go". In this case, can I still say "He told me that he should have gone to school yesterday"? Or do I have to say "He told me that he had to go to school yesterday"?


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## Logos_

grammar-in-use said:


> I wanted to convey "he did go". In this case, can I still say "He told me that he should have gone to school yesterday"? Or do I have to say "He told me that he had to go to school yesterday"?


"Had to go" "He told me he had to go to school yesterday, so he couldn't come to the party."


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## grammar-in-use

Logos_ said:


> "Had to go" "He told me he had to go to school yesterday, so he couldn't come to the party."


Yes, that's how I normally understand it.

Then does it make sense to say "He told me he *should* *have* *gone* to school yesterday, so he couldn't come to the party."?

What about "He told me he *should* *have* *gone* to school yesterday, but still he came to the party."?


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## Logos_

_"He told me he *should* *have* *gone* to school yesterday, so he couldn't come to the party."_

I would avoid that. It sounds strange and ambiguous.

_ "He told me he *should* *have* *gone* to school yesterday, but still he came to the party."_

That would be acceptable but then you would probably have to go on and explain why he was able to come to the party despite the fact that he should have gone to school.


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## lingobingo

grammar-in-use said:


> Please can you refer to post #2, which I'd understand it as acceptable (if I understand correctly).


Sorry – I didn’t read the earlier posts. 
It’s true that you can use *must* in reported speech to mean “need to”, but _only_ when referring to the present or future within that time frame.

“I must deal with it,” he said. (talking about a future intention)​He said he must deal it.  / He told me he needed to deal with it. ​
But you can’t restrict it to a specific time in the past:

“I need to go to school today,” he told me last Friday.​He told me that he must go to school last Friday. ​He told me he had to go to school last Friday. ​He told me last Friday that he had to go to school that day. ​


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## grammar-in-use

Logos_ said:


> _"He told me he *should* *have* *gone* to school yesterday, so he couldn't come to the party."_
> 
> I would avoid that. It sounds strange and ambiguous.
> 
> _ "He told me he *should* *have* *gone* to school yesterday, but still he came to the party."_
> 
> That would be acceptable but then you would probably have to go on and explain why he was able to come to the party despite the fact that he should have gone to school.


OK, I see. Thank you.
Would you understand "He told me he *must* go to school yesterday" as "should have gone" or "had to go"?


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## tunaafi

I agree with what lingobingo said in post #25. 

Having read the whole thread, I realise that my post #2 was unhelpful (to say the least). My apologies. I considered deleting it, but, as other members have referred to it, I'll let it stand.


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## Logos_

grammar-in-use said:


> OK, I see. Thank you.
> Would you understand "He told me he *must* go to school yesterday" as "should have gone" or "had to go"?



I would avoid that and rephrase it. But if someone said it I would understand it as "had to go."


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## grammar-in-use

lingobingo said:


> It’s true that you can use *must* in reported speech to mean “need to”, but _only_ when referring to the present or future within that time frame.


Crystal clear! Thank you very much for your illuminating examples and explanation!

Let me change to another example. Does it make sense to say:
We *knew* that we *must* set off *by noon yesterday* to arrive home before sunset.
?

But we still can't just say "we *must* set off *by noon yesterday* to arrive home before sunset", right?


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## Logos_

grammar-in-use said:


> We *knew* that we *must* set off *by noon yesterday* to arrive home before sunset.
> ?



Had to would be much more idiomatic in relating past events.


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## grammar-in-use

Logos_ said:


> Had to would be much more idiomatic in relating past events.


Yeah, I know. It's comparatively easy to use "had to" and "should have done". But I'm just trying to check the use of "must" and the extent to which it can be compatible with the past time.


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## Logos_

"we *must* set off *by noon yesterday* to arrive home before sunset"

I would say "We _would have had to_ set off by noon yesterday to arrive home before sunset." Or simply "We had to set off by..." depending on the meaning you wanted to convey.

 "Must" wouldn't work there.


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## grammar-in-use

Logos_ said:


> "we *must* set off *by noon yesterday* to arrive home before sunset"
> 
> I would say "We _would have had to_ set off by noon yesterday to arrive home before sunset." Or simply "We had to set off by..." depending on the meaning you wanted to convey.
> 
> "Must" wouldn't work there.


Yeah, I agree.

We _*would have had* to_ set off by noon yesterday to arrive home before sunset.
This means that we didn't actually set off by noon yesterday and that we arrived home later than expected, right?


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## Logos_

grammar-in-use said:


> This means that we didn't actually set off by noon yesterday and that we arrived home later than expected, right?



Yes. If you did set off at that time, you would write "we had to set off...etc."


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## grammar-in-use

Logos_ said:


> Yes. If you did set off at that time, you would write "we had to set off...etc."


Thanks a lot for confirming it.


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## lingobingo

grammar-in-use said:


> Does it make sense to say:
> We *knew* that we *must* set off *by noon yesterday* to arrive home before sunset.
> 
> But we still can't just say "we *must* set off *by noon yesterday* to arrive home before sunset", right?


Yes, that’s right – in principle. You’re using *must* to talk in the past tense about (what was at that time) a future necessity. Even so, I don’t think any writer worth his or her salt  would use it with such a specific and recent time marker as the word “yesterday”!
​We knew that we must set off by noon *yesterday* to arrive home before sunset.​We knew *at the time* that we must set off by noon *if we were* to arrive home before sunset. ​


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## grammar-in-use

lingobingo said:


> Yes, that’s right – in principle. You’re using *must* to talk in the past tense about (what was at that time) a future necessity. Even so, I don’t think any writer worth his or her salt  would use it with such a specific and recent time marker as the word “yesterday”!
> ​We knew that we must set off by noon *yesterday* to arrive home before sunset.​We knew *at the time* that we must set off by noon *if we were* to arrive home before sunset. ​


OK, I completely understand now. Thank you very much! You've always been so helpful, with illuminating examples and clear and concise explanation!


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## Thomas Tompion

handsomechuck said:


> [...]
> Must can be used in the past. Sometimes, especially in (older) writing, where we would use "had to," you will see something like "The building was on fire. He must escape or perish."


I don't think this is a use of 'must' with past force.  It's a historic present, use of the present tense in the narration of a past event.


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## Steven David

grammar-in-use said:


> Hello everyone,
> 
> It'd be nonsensical to say "He *must* go to school *yesterday*", right? Then what about "He *told* me that he *must* go to school *yesterday*"?
> The latter seems to be acceptable, right? So, too, is "He *had* to go to school yesterday".
> 
> Could some native speakers shed some light on the three sentences above? I'd really appreciate it.




Of course, the idea is that he told you yesterday. 

Yesterday, he told me that he must go to school. 

He told me, yesterday, that he must go to school. 

He told me that he must go to school yesterday. < This one sounds strange because "yesterday" is not close enough to the verb "told". There's too much separation between "told" and "yesterday". 

Modal verb "must" can only refer to present or future time. So the sentences means that, in general, he must go to school, and he told you this yesterday. 

With "have to", there's more flexibility with where we can place "yesterday" because there's a past form of 'have to", which is "had to", of course. 

1) Yesterday, he told me that he had to go to school. 

2) He told me, yesterday, that he had to go to school. 

3.1) He told me that he had to go to school yesterday. < 1) Yesterday was the day he had to go to school. < The means the specific time he told you. 

3.2) He told me that he had to go to school yesterday. < 2) He told me, yesterday that he had to go to school, and he's starting classes this winter. < This means that, in general, going to school is something he has to do, and he told you this yesterday. 

There's some ambiguity with the last sentence, number 3,. However, the meaning is made clear in context. 

Steve


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## grammar-in-use

Thank you very much, Steve!



Steven David said:


> Modal verb "must" can only refer to present or *future* time.


This comment of yours reminds me of whether "must" can be used to make *predictions*, as in:
a. According to the forecast, it *must* be warm *tomorrow*.

Obviously, "tomorrow" is a future time, but can I say sentence (a) above?

To compare:
b. According to the forecast, it *should* be warm *tomorrow*.

Also, can I say:
c. He must be leaving tomorrow.
??  Is it a prediction or a necessity for him to leave tomorrow?


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## Logos_

Sentence b is much more idiomatic. Of course, you could also say, "According to the forecast, it will be warm tomorrow."


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## Logos_

The problem is that "must" implies that it could not be anything other than warm, but the whole reason for the existence of a forecast is the fact that it indeed _can_ be something other than warm. In other words, must would imply that the weather is _required_ to be warm tomorrow, which is a very odd, rigidly deterministic way of thinking about the weather.

However, we could say "The water must reach 32 degrees Fahrenheit before it will freeze." This is because this a law of natural science. It cannot be otherwise. In all (ordinary) cases, it _must _be so before the water will freeze.


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## grammar-in-use

Logos_ said:


> Sentence b is much more idiomatic. Of course, you could also say, "According to the forecast, it will be warm tomorrow."


Thank you!
Can I use "have/has to", replacing "must", to make predictions, as in:
a1. According to the forecast, it *has to* be warm *tomorrow*.
??


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## Logos_

grammar-in-use said:


> Thank you!
> Can I use "have/has to", replacing "must", to make predictions, as in:
> a1. According to the forecast, it *has to* be warm *tomorrow*.
> ??


I address this somewhat in post #42. Because we normally think of the weather as undetermined, volatile, and subject to continual change, it would be unusual to think of it has "having to be" a certain way or "obliged to be" another way. Instead, we simply say what it will be (in all likelihood). That is, we don't consider it to be a fixed natural law that it will be warm tomorrow, although it almost certainly will be warm. This is informed by the common sense view of weather as opposed to a strictly deterministic view that we might take if examining things in a purely scientific way.


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## grammar-in-use

Logos_ said:


> The problem is that "must" implies that it could not be anything other than warm, but the whole reason for the existence of a forecast is the fact that it indeed _can_ be something other than warm. In other words, must would imply that the weather is _required_ to be warm tomorrow, which is a very odd, rigidly deterministic way of thinking about the weather.
> 
> However, we could say "The water must reach 32 degrees Fahrenheit before it will freeze." This is because this a law of natural science. It cannot be otherwise. In all (ordinary) cases, it _must _be so before the water will freeze.


It does make sense!

Then can I say:
c. He *must be leaving* tomorrow.
?? Is it a prediction or a necessity for him to leave tomorrow?


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## Logos_

grammar-in-use said:


> c. He *must be leaving* tomorrow.
> ?? Is it a prediction or a necessity for him to leave tomorrow?


That would usually mean, "He has an obligation that requires him to leave tomorrow." It necessitates his departure tomorrow.

If you say "I must be going" it means I have to go because I need to get some sleep or I have another important errand or a meeting, etc. "I have enjoyed the party but I must be going since I have a doctor's appointment."


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## grammar-in-use

Logos_ said:


> That would usually mean, "He has an obligation that requires him to leave tomorrow."
> 
> If you say "I must be going" it means I have to go because I need to get some sleep or I have another important errand or a meeting. "I have enjoyed the party but I must be going since I have a doctor's appointment."


OK, I see. And I also agree. But, one of my American teachers (who is also very good at the Chinese language) once told me that it would mean something like "In my opinion, he will probably leave tomorrow", which I think is still a prediction interpretation, isn't it?


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## Logos_

grammar-in-use said:


> "In my opinion, he will probably leave tomorrow", which I think is still a prediction interpretation, isn't it?



It is, but that is not the meaning of the sentence _itself_. It is a further prediction you could make _from _the sentence. The sentence means that he is obliged to leave tomorrow. With that information, we can _predict_ that he will probably or almost certainly leave tomorrow.


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## grammar-in-use

Logos_ said:


> It is, but that is not the meaning of the sentence _itself_. It is a further prediction you could make _from _the sentence. The sentence means that he is obliged to leave tomorrow. With that information, we can _predict_ that he will probably or almost certainly leave tomorrow.


OK, I see. The last question, does it sound natural to you to say "he must be leaving tomorrow"?


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## Logos_

grammar-in-use said:


> OK, I see. The last question, does it sound natural to you to say "he must be leaving tomorrow"?


Yes, and the meaning would be that he is obliged or required to depart tomorrow for some reason such as another engagement.


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## grammar-in-use

Logos_ said:


> Yes, and the meaning would be that he is obliged or required to depart tomorrow for some reason such as another engagement.


Great! Thank you very much.


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## Thomas Tompion

grammar-in-use said:


> OK, I see. The last question, does it sound natural to you to say "he must be leaving tomorrow"?


In British English this is something we could easily say, indicating that we can infer from his behaviour - we've seen him packing his bags - or some other indication - his wife has cancelled the daily delivery of milk - that he is leaving tomorrow.


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## grammar-in-use

Thomas Tompion said:


> In British English this is something we could easily say, indicating that we can infer from his behaviour - we've seen him packing his bags - or some other indication - his wife has cancelled the daily delivery of milk - that he is leaving tomorrow.



Aha, your interpretation of "he must be leaving tomorrow" seems to be different, if I understand it correctly, from Logos's, as in:


Logos_ said:


> That would usually mean, "He has an obligation that requires him to leave tomorrow." It necessitates his departure tomorrow.



In yours, it is what I'd like to call "*prediction* *must*", whereas in Logos's, it is "obligation must" or "*necessity* *must*". What do you think?


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## Thomas Tompion

grammar-in-use said:


> Aha, your interpretation of "he must be leaving tomorrow" seems to be different, if I understand it correctly, from Logos's, as in:
> 
> 
> In yours, it is what I'd like to call "*prediction* *must*", whereas in Logos's, it is "obligation must" or "*necessity* *must*". What do you think?


Yes, it is different; that's why I mentioned it.  They are not mutually inconsistent, of course.

Certainly, in the sense I mentioned, we are drawing an inference about his likely future behaviour from what's been going on recently, and he may be under no obligation at all to leave tomorrow.

In British English we'd often distinguish between the two by altering the form of the verbs:

_He must leave tomorrow _- he's obliged to leave tomorrow.
_He must be leaving tomorrow_ - it seems likely that he plans to leave tomorrow.


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## grammar-in-use

Thomas Tompion said:


> In British English we'd often distinguish between the two by *altering the form of the verbs*:
> 
> _a. He must leave tomorrow _- he's obliged to leave tomorrow.
> _b. He must be leaving tomorrow_ - it seems likely that he plans to leave tomorrow.


Thank you for confirming it.
A couple of queries have come to mind:
1. Can sentence (a) be understood as a prediction about his departure tomorrow?
2. As far as I know, we don't normally use "must" to make predictions, and that's why we don't say, for example, "According to the forecast, it *must* be warm *tomorrow*", as we discussed in posts #40, 41 and 42. Why, in sentence (b), can we use "must" together with "tomorrow" to draw "an inference about his likely future behaviour"?


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## Thomas Tompion

grammar-in-use said:


> 1. Can sentence (a) be understood as a prediction about his departure tomorrow?


Not usually in British English.


grammar-in-use said:


> 2. As far as I know, we don't normally use "must" to make predictions, and that's why we don't say, for example, "According to the forecast, it *must* be warm *tomorrow*", as we discussed in posts #40, 41 and 42. Why, in sentence (b), can we use "must" together with "tomorrow" to draw "an inference about his likely future behaviour"?


The fact is that we do; how the convention became established is beyond my scope.


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## grammar-in-use

Thomas Tompion said:


> The fact is that we do;


I doubt if you (British people) would say "According to the forecast, it *must* be warm *tomorrow*". Does it sound natural, with "must" used to make a prediction about tomorrow's weather?


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## Thomas Tompion

grammar-in-use said:


> I doubt if you (British people) would say "According to the forecast, it *must* be warm *tomorrow*". Does it sound natural, with "must" used to make a prediction about tomorrow's weather?


We'd say _According to the forecast, it should be warm tomorrow_, but we could say something like _Given the atmospheric pressure and the weather these last few days, it really must be warm tomorrow._


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## grammar-in-use

Thomas Tompion said:


> We'd say _According to the forecast, it should be warm tomorrow_, but we could say something like _Given the atmospheric pressure and the weather these last few days, it really must be warm tomorrow._


OK, I see. Thank you very much!


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## lingobingo

Lexico only gives, as contexts in which *must* is used: 1. obligation/necessity, 1.1 insistence, 1.2 irritation, and 2. opinion (“about something that is logically very likely”). 

I agree that there is no ‘prediction’ use, as such. The modal verbs that express prediction are will/would, should and could. 

It’s possible to cite constructions in which the use of *must* appears to indicate a prediction, but in fact it’s always the context that does that, rather than the word itself. All *must* can be said to express in such an example is a deduction, which falls into category 2: opinion.

This can perhaps be seen in the phrase “That must be it!” (Eureka!). We make an assumption or deduction and explain it by implying that it is, of necessity, true. But although we express it as though there can be no doubt, what we really mean is “there’s no doubt in _my_ mind”. It’s my opinion/belief.


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## grammar-in-use

lingobingo said:


> I agree that there is no ‘prediction’ use, as such.



Thank you for confirming that.

But "must" can be used for the present deduction, right? Like:

c. The building must be demolished.

But on second thoughts, sentence (c) can be ambiguous between a *future event* of "obligation" and a *present state* of "deduction", right?


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## Thomas Tompion

lingobingo said:


> [...]
> This can perhaps be seen in the phrase “That must be it!” (Eureka!). We make an assumption or deduction and explain it by implying that it is, of necessity, true. But although we express it as though there can be no doubt, what we really mean is “there’s no doubt in _my_ mind”. It’s my opinion/belief.


What about when a hypothesis is extensively tested empirically, to the point of becoming a theory.  Surely that takes us out of the realm of personal opinion.


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## lingobingo

grammar-in-use said:


> But "must" can be used for the present deduction, right? Like:
> 
> c. The building must be demolished.


That’s only a deduction if presented as such – in relation to whatever fact leads to that conclusion. As it stands, it’s a simple statement of necessity/inevitability (we are obliged to demolish it).


Thomas Tompion said:


> What about when a hypothesis is extensively tested empirically, to the point of becoming a theory. Surely that takes us out of the realm of personal opinion.


I take it you mean when *must* is used to aid a statement to the effect that – on the basis of knowing, from experience, how these things pan out – a certain thing will inevitably happen? If so, that might indeed enter a different realm from that of trite personal opinions. But the word *must* would still not, in itself, denote prediction. The element of prediction would rely on the context.


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## Thomas Tompion

lingobingo said:


> [...]





lingobingo said:


> I take it you mean when *must* is used to aid a statement to the effect that – on the basis of knowing, from experience, how these things pan out – a certain thing will inevitably happen? If so, that might indeed enter a different realm from that of trite personal opinions. But the word *must* would still not, in itself, denote prediction. The element of prediction would rely on the context.


So you are saying that when a scientist uses 'must' to predict an outcome, based on empirical observation and a well established law, he would not be expressing a private opinion.


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## lingobingo

Am I?

I thought I was saying that the modal verb *must* is incapable in itself of implying prediction.


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## Packard

I would probably say "had to" like many of the above, but "was required to" would work also.

_He was required to go to school yesterday._


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## Thomas Tompion

lingobingo said:


> Am I?
> 
> I thought I was saying that the modal verb *must* is incapable in itself of implying prediction.


I'm sorry; I wasn't trying to put words into your mouth, or saying you were necessarily wrong.

I was wondering how your statement squared with the scientists' use of 'must' - eg. _When you take the boat out of the lake, the level of water at the edge of the lake must fall_. That's hardly a statement of opinion.


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## lingobingo

I’m happy to withdraw my last paragraph in #60, which I should probably have deleted (having edited what I said before it). That example is misleading. I don’t believe that the “opinion” definition of *must* always applies to that sort of example.


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## Packard

Thomas Tompion said:


> I'm sorry I wasn't trying to put words into your mouth, or saying you were necessarily wrong.
> 
> I was wondering how your statement squared with the scientists' use of 'must' - eg. _When you take the boat out of the lake, the level of water at the edge of the lake must fall_. That's hardly a statement of opinion.


It is an equation.  (If you accept that Archimedes Principle is an equation.)  

_If you add two to any number the result must be larger than the original by two.  So two plus six* must *equal eight._


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## Steven David

grammar-in-use said:


> Thank you very much, Steve!
> 
> 
> This comment of yours reminds me of whether "must" can be used to make *predictions*, as in:
> a. According to the forecast, it *must* be warm *tomorrow*.
> 
> Obviously, "tomorrow" is a future time, but can I say sentence (a) above?
> 
> To compare:
> b. According to the forecast, it *should* be warm *tomorrow*.
> 
> Also, can I say:
> c. He must be leaving tomorrow.
> ??  Is it a prediction or a necessity for him to leave tomorrow?



You're welcome!

I'll use a couple technical words. We should differentiate between epistemic must and deontic must.

With deontic must, we speak of obligations and social expectations.

With epistemic must, which is relevant to your question here, we speak of predictions based on evidence and what is in the environment, circumstance, or situation. This use of must also is based on our understanding of our world. Also, epistemic has to do with how we interpret things in our environment. So it's not just about predictions. It's also about what we believe is true based on our knowledge and understanding of our environment aTnd the world.

Sentence a, no, does not work. The forecast is not enough for it to be a strong enough prediction that the weather will be warm tomorrow. By contrast, on the other hand, the forecast is, as we understand it, something given with a great deal of certainty, which is more certainty than epistemic must can provide.

Sentence b makes sense and is, yes, possible. Epistemic should works well with a weather prediction or forecast. Epistemic should is not as strong as epistemic must.

Sentence c is ambiguous. It could go either way depending on context. However, without context, I take this sentence to mean that it's likely that he's going to be leaving tomorrow. So I read the sentence as epistemic, which is to say epistemic must.

Here is another example of the same ambiguity. The sentence can be read as obligation and requirement or what we understand to be true based on our knowledge of the world.

You must work very hard to be a chef that owns a restaurant, manages a restaurant, and works as the head chef of a restaurant.

I understand this to be true. Also, this is an obligation or requirement.

If someone says that he or she wants to open a restaurant and be the chef and manage the restaurant, then the obligation and the requirement interpretation would be a reply to someone who says this. This would be advice or guidance, and, in this way, this use of must would be deontic.


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## grammar-in-use

Thank you very much, Steve!


Steven David said:


> We should differentiate between *epistemic must* and *deontic must*.
> 
> With deontic must, we speak of obligations and social expectations.
> 
> With epistemic must, which is relevant to your question here, we speak of predictions based on evidence and what is in the environment, circumstance, or situation. This use of must also is based on our understanding of our world. Also, epistemic has to do with how we interpret things in our environment. So it's not just about predictions. It's also about what we believe is true *based on our knowledge* and understanding of our environment aTnd the world.


So, I'd like to summarize your "epistemic must" as *knowledge-based*, and "deontic must" as *socially based*.

With your example:


Steven David said:


> a. You *must work *very hard to be a chef that owns a restaurant, manages a restaurant, and works as the head chef of a restaurant.


what if I say:
b. You *must be working* very hard to...
?

Is the "must" in sentence "b" (normally understood as) knowledge-based or socially based or either?


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## Thomas Tompion

grammar-in-use said:


> Thank you very much, Steve!
> 
> So, I'd like to summarize your "epistemic must" as *knowledge-based*, and "deontic must" as *socially based*.
> 
> With your example:
> 
> what if I say:
> b. You *must be working* very hard to...
> ?
> 
> Is the "must" in sentence "b" (normally understood as) knowledge-based or socially based or either?


Oh dear! We seem to have opened a can of worms here.

b. shows little sense of moral obligation that I can see. _ 

Deontic_ means pertaining to duty or obligation - eg. _we must be kind to our parents_.


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## grammar-in-use

Thomas Tompion said:


> b. shows little sense of moral obligation that I can see.



With the same pattern "must + be doing" as "he *must be working* very hard", why can "he *must be leaving* tomorrow" be *interpreted deontically*? As shown in post #46:



Logos_ said:


> That would usually mean, "He has an obligation that requires him to leave tomorrow." It necessitates his departure tomorrow.


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## Thomas Tompion

grammar-in-use said:


> With the same pattern "must + be doing" as "he *must be working* very hard", why can "he *must be leaving* tomorrow" be *interpreted deontically*? As shown in post #46:


I agree that it could just be c_. He *must be leaving* tomorrow - he is obliged to leave tomorrow_.

All would depend on the context: it could mean _We can reasonably infer from the indications that he will be leaving tomorrow_, ie. no sense of obligation at all.

One of the charms (and difficulties) of these modal auxiliaries is that they can have different meanings.


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## Steven David

grammar-in-use said:


> Thank you very much, Steve!
> 
> So, I'd like to summarize your "epistemic must" as *knowledge-based*, and "deontic must" as *socially based*.
> 
> With your example:
> 
> what if I say:
> b. You *must be working* very hard to...
> ?
> 
> Is the "must" in sentence "b" (normally understood as) knowledge-based or socially based or either?




You're welcome. 

Yes, that's a good summary statement. 


Sentence b. Without context, I read it as knowledge-based. 

The idea that it is in progressive form also tells us that this is something that is happening "around now", so this is something that the speaker perceives. Perception has to do with what we believe we know and understand, so, again, without context, sentence b comes across as knowledge-based. 

Context could change the perception of sentence b. 

epistemic is a rather lofty word, some might say, for this context. However, I like, it makes sense to apply it here. 

As I understand it, I would summarize epistemology as follows: This has to do with whether or not "we really know what we know, and how we can be sure that what we know is reality".


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## grammar-in-use

Steven David said:


> You're welcome.
> 
> Yes, that's a good summary statement.
> 
> 
> Sentence b. Without context, I read it as knowledge-based.
> 
> The idea that it is in progressive form also tells us that this is something that is happening "around now", so this is something that the speaker perceives. Perception has to do with what we believe we know and understand, so, again, without context, sentence b comes across as knowledge-based.
> 
> Context could change the perception of sentence b.
> 
> epistemic is a rather lofty word, some might say, for this context. However, I like, it makes sense to apply it here.
> 
> As I understand it, I would summarize epistemology as follows: This has to do with whether or not "we really know what we know, and how we can be sure that what we know is reality".



Thank you, Steve!


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