# "couldn't" vs "could not"



## Ariel Knightly

Is there any difference in meaning between these?

(1) _She couldn't have seen you.
_(2)_ She could not have seen you._

I think 1 means something like "It was impossible for her to see you," and 2 something like "Maybe she didn't see you." But as my textbook says _couldn't have_ can express both uncertainty and possibility, I'm not really sure I'm right about this contrast I just made. I'm also quite confused because 1 was taken from this exercise I found in another textbook:

_I passed Mary in the street and she didn't speak to me.
a) She mustn't have seen you.
b) She couldn't have seen you.

_To me, both options sound unlikely - I'm actually under the impression that a is even ungrammatical. I would have picked _She must not have seen_ _you _or _She could not have seen you_. What do you guys think about it?

Thank you very much in advance!


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## Miss Julie

"Couldn't" is just a contraction of "could not." They mean the same thing.

"She mustn't (must not) have seen you" is completely grammatical. Again, "mustn't" is just the contraction of "must not." No difference in meaning.


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## JustKate

_Couldn't_ and _could not_ mean the same thing. The only difference is that the two-word version _could not_ often has the effect of really emphasizing that _not_. If it _could not_ is spoken rather than written, a native speaker would invariably put quite a bit of emphasis on _not_. 

As for (a) and (b) both are possible and correct. The only problem is that in AmE, we just don't use _mustn't_ very often these days. There's no real reason for it at least not as far as I know; that's just the way it is.

(Cross-posted with Miss Julie)


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## TheKingOfSpain

First of all, just as a quick note: At least in American/Canadian English I think it is quite rare to hear mustn't when talking to anybody under the age of 50. It's still used rarely, but in your example with Mary I would say "must not have seen" instead.

I think you are being tripped up by the fact that "could have" is a past form of can, but can also express uncertainty and possibility.

Some examples:
"I could climb that wall." (present, conditional) This sentence expresses a possibility RIGHT NOW.
"I could have climbed that wall." (past, conditional) This sentence expresses a possibility that USED TO EXIST, but probably does not now.

"I could not have climbed that wall." (past, possibly conditional) This sentence depends more on context. It could mean "I would not have been able to climb that wall" FOLLOWED BY "if .. [some condition]". This means you DID climb the wall, but if something else were true, that would not have been possible. The other possibility is you may be remarking at someone else who climbed the wall, saying that you would not be able to do what that person did. These examples are similar to the "could not's/couldn't's" that you have listed above. In those cases however, the possibility does not exist because those sentences are standalone sentences with no "if", so the first possibility doesn't apply.

The Final Possibility (aka the answer you're looking for, I think):
"I could have not climbed that wall" (past, conditional) This sentence simply means that you did climb the wall, but you did not need to. The confusing thing here is that what you would have been able to do is the negation of something.

In conclusion, these sentences have the following meanings:

"She mustn't have seen you/She must not have seen you" - By chance, she didn't see you. There was the possibility that she could have seen you, but she didn't.
"She couldn't have seen you" - It was impossible for her to see you.
"She could have not seen you" - MAYBE she didn't see you.

Hopefully that helps, and if anyone disagrees, I would love more input, because this is just off the top of my head.


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## boozer

Ariel Knightly said:


> (1) _She couldn't have seen you.
> _(2)_ She could not have seen you._
> 
> I think 1 means something like "It was impossible for her to see you," and 2 something like "Maybe she didn't see you."...


Yes, I think this is a possible interpretation for 2. I could imagine interpreting it this way if it was said in a suitable manner, the logical stress falling on the word 'could' and the rest of the sentence being said quicker as a discrete utterance within the sentence:
She *could *|not have seen you|. 

But this would be an exotic way of expressing the idea. I would, under normal circumstances, see no difference at all between 1 and 2. 

Well, that is my opinion, for what it's worth...


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## TheKingOfSpain

Boozer, I see what you're getting at. While this may be applicable in some situations, I think what I posted is better on the whole: "She could have not seen you." expresses exactly what you're getting at with no weirdness.


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## boozer

TheKingOfSpain said:


> Boozer, I see what you're getting at. While this may be applicable in some situations, I think what I posted is better on the whole: "She could have not seen you." expresses exactly what you're getting at with no weirdness.


Oh, but I have no quarrel with that - you are right.  However, Ariel Knightly was asking if his interpretation was possible and I think that it is, indeed, possible.


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## TheKingOfSpain

Indeed. It's just hard to communicate how voice inflection is supposed to change over the internet, hehe.


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## kalamazoo

I don't think this is a clear-cut situation.  Depending on context and inflection, the meaning could be either uncertainty or (lack of) possibility.


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## Ariel Knightly

boozer said:


> Yes, I think this is a possible interpretation for 2. I could imagine interpreting it this way if it was said in a suitable manner, the logical stress falling on the word 'could' and the rest of the sentence being said quicker as a discrete utterance within the sentence:
> She *could *|not have seen you|.


Okay, She could |not have seen you| and She could NOT have seen you have two different meanings, right? Am I wrong when I say that the former expresses uncertainty and the latter impossibility? What about She couldn't have seen you? Can it mean both or can couldn't only be synonymous with could NOT?


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## kalamazoo

I don't think you can say definitively without some context.  "She couldn't have seen you at the cafe yesterday because she was in New York at the time" and "She could not have seen you at the cafe yesterday because she was in New York at the time" both mean exactly the same thing - impossibility.  But in the dialogue "She passed me on the street yesterday and didn't even say hello" "She could not have seen you" it means "it's possible that she didn't see you."


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## TheKingOfSpain

Ariel Knightly said:


> Okay, She could |not have seen you| and She could NOT have seen you have two different meanings, right? Am I wrong when I say that the former expresses uncertainty and the latter impossibility? What about She couldn't have seen you? Can it mean both or can couldn't only be synonymous with could NOT?



"Couldn't" can only be synonymous with "could NOT".


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## Ariel Knightly

kalamazoo said:


> But in the dialogue "She passed me on the street yesterday and didn't even say hello" "She could not have seen you" it means "it's possible that she didn't see you."





TheKingOfSpain said:


> "Couldn't" can only be synonymous with "could NOT".


Then b would only make sense if the woman were blind, right? She could |not have seen you| would be much more likely in this context, wouldn't it?

_I passed Mary in the street and she didn't speak to me.
a) She mustn't have seen you.
b) She couldn't have seen you.
_
What about mustn't have? Would there be any difference between She mustn't have seen you and She must |not have seen you|?


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## kalamazoo

What is the question here?  I think somehow you are trying to draw a bright line where there isn't one. 


- I passed Mary in the street and she didn't speak to me
- Maybe she didn't see you/it's possible she didn't see you/she must have been distracted and not noticed you/she might not have seen you/she could not have seen you/she could have not seen you

All are about the same.


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## Forero

_She couldn't have seen you._
= She could not possibly have seen you.

_She *could* not have seen you._
= She could possibly not have seen you.

_She could *not* have seen you._
= Either of the above (ambiguous), but with different emphasis.

I don't see any ambiguity between _mustn't have_ and _must not have_.


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## Ariel Knightly

Kalamazoo, I'm actually just trying to understand that exercise I saw in my textbook. The key says that_ b_ is the correct answer (She couldn't have seen you), but you guys have apparently just confirmed that could |not| and couldn't (could NOT) have two different meanings, which makes_ b_ an unlikely option, doesn't it?


kalamazoo said:


> What is the question here?


*(1)* Is the answer key really wrong?

Michael Swan says in his _Practical English Usage_ that "[w]e can use *must have + past participle* to express certainty about the past." He also says that it's can the form used in questions and negatives. And these are his examples:

_We went to Rome last month. ~ That *must have been* nice.
Where *can *John *have put* the matches? ~ He *can't have thrown* them away._

But then he says that, "_n American English, must not is often used when something is not logically impossible, but when there is strong evidence for believing that it is not the case." 

She walked past without saying 'Hello'. She *must not* have seen you. (AmE; some British speakers.)
She walked past without saying 'Hello'. She *can't *have seen you. (most British speakers.)

Then he tells us to note that "the contracted form musn't is rare in AmE." 


kalamazoo said:



			What is the question here?
		
Click to expand...

*(2) *Would you native speakers really have used that contracted form in a? Or would must not have sound more natural?


Forero said:



			I don't see any ambiguity between mustn't have and must not have.
		
Click to expand...

But do both forms sound perfectly natural?_


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## boozer

Okay, here's the deal  :

_1. She couldn't have seen you. = It is not possible for her to have seen you
2. She could not have seen you. = It is not possible for her to have seen you
3. She COULD |not have seen you|. = It is possible that she did not see you._ 

Please note that the only difference between 2 and 3 is in the way they are pronounced to modify the meaning and, as TheKingOfSpain points out, it is very difficult to explain that in a forum when we have to write, as opposed to speaking.


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## Ariel Knightly

Thank you guys very much! Now I think I can understand it better. 

_I passed Mary in the street and she didn't speak to me._
_a) She mustn't have seen you. _[not as likely as She must not have seen you, for the contracted form is not so common in AmE.]
_b) She couldn't have seen you. _[unlikely, for it doesn't mean the same as She COULD not have seen you, which would make much more sense in this context.]


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## JustKate

Boozer is right, but I'd just like to add that (1) and (2) are by *far* the more common ways to interpret these sentences. (3) is possible, but it's really only possible in speech. If someone was trying to convey this meaning in writing they'd have to change the sentence because almost nobody (including native speakers) would understand. This particular meaning depends entirely on the emphasis that can be conveyed only verbally.


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## Sparky Malarky

I agree with everyone (I think).  But it's also possible to say "she couldn't have seen you," meaning literally* it is impossible for her to have seen you*, while still meaning figuratively *it is possible that she did not see you*. 

I passed Mary on the street and she didn't speak to me.
Oh, she couldn't have seen you. _(Surely she didn't snub you; Mary wouldn't do that.  It must be that she didn't see you.)_


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## Ariel Knightly

I see. You would be saying it is impossible for her to have seen him because you know that that kind of rudeness is not a possible behavior for her, right?


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## Forero

"Mustn't have + p.p." is not common where I live, but I think I understand what it means.


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## boozer

Forero said:


> "Mustn't have + p.p." is not common where I live, but I think I understand what it means.


It is not something I say every day either. In fact, I don't think I ever say it at all, which is why so far I have masterfully managed to dodge this topic.  However, I suppose it could have the same exotic meaning as 'could' when said with a suitable intonation:

_She must |*not *have seen you|. - I am pretty sure she did not see you.
She mustn't have seen you.  - you cannot prohibit something that has already come to pass..._


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## TheKingOfSpain

boozer said:


> It is not something I say every day either. In fact, I don't think I ever say it at all, which is why so far I have masterfully managed to dodge this topic.  However, I suppose it could have the same exotic meaning as 'could' when said with a suitable intonation:
> 
> _She must |*not *have seen you|. - I am pretty sure she did not see you.
> She mustn't have seen you.  - you cannot prohibit something that has already come to pass..._



"She must |not have seen you|" does not work like "She could |not have seen you|". It doesn't matter how you say it, all these sentences with different pronunciations and constructions surrounding "must not" all have the same meaning. It simply means "(The only possible explanation is that) She didn't see you". i.e. everything else is unfathomable, so the final alternative MUST be true. With could |not have seen you| you are saying that in the past, there was the possibility (she had the ability) to not see you. It could have happened, that she did not see you.

Must not can only be meant one way, unless I am very much incorrect.

Edit: Here are some examples, hopefully to give a definitive picture of the options:

Example 1: could |not have seen|
A- Hey. I was walking down the street and I saw Macy on the other side of the road, getting into a cab. I shouted to her, and I'm pretty sure she looked at me, but she ignored me.


B- Aww man, that sucks. Don't take it too badly though, [she could |not have seen you|/she could have not seen you].


Example 2: couldn't have seen/could NOT have seen
A- Hey. I was walking down the street and I saw Macy on the other side of the road, getting into a cab. I shouted to her, and I'm pretty sure she looked at me, but she ignored me.


B- Naw, she didn't ignore you. She couldn't/could not have heard you over the noise of everyone else on the street. 
(This is an unaltered version of the phrase, with an explanation tacked on to the end. A better sounding version might go like this: "She couldn't have possibly heard you over the noise..." or "There's no way she could have heard you over the noise...")


Example 3: must not have seen
A- Hey. I was walking down the street and I saw Macy on the other side of the road, getting into a cab. I shouted to her, and I'm pretty sure she looked at me, but she ignored me.


B- Don't be ridiculous. You and Macy are really good friends. That's not like her to ignore you. She must not have seen you.


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## boozer

TheKingOfSpain said:


> ...
> B- Don't be ridiculous. You and Macy are really good friends. That's not like her to ignore you. _She must not have seen you_.


King, your example works exactly the way I thought it would = I am pretty sure she did not see you. 
In this example given by you, would you say She mustn't have seen you instead? I surely wouldn't


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## TheKingOfSpain

boozer said:


> King, your example works exactly the way I thought it would = I am pretty sure she did not see you.
> In this example given by you, would you say She mustn't have seen you instead? I surely wouldn't



Mustn't and must not have the EXACT same meaning. However, mustn't is not used very frequently in AmE. It sounds formal and old, I think. It's the kind of thing that I would not find weird coming from my grandmother, for example.


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## boozer

I see. For me 'She mustn't have seen you' makes no sense at all. For me 'mustn't' is prohibition.  We seem to agree on the meaning of 'She must not have seen you' though.


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## TheKingOfSpain

Well, as a final note, I'm not sure that mustn't has ever been used in a situation like this. It would always be must not in practice. The "more common" usage of mustn't is in phrases like "You mustn't do that". However, the fact remains that mustn't is a contraction of must not, and provided that the use of the contraction doesn't imply some other meaning (which it doesn't) the two must be equivalent, as far as I know.


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## boozer

TheKingOfSpain said:


> Well, as a final note, I'm not sure that mustn't has ever been used in a situation like this. It would always be must not in practice. The "more common" usage of mustn't is in phrases like "You mustn't do that". However, the fact remains that mustn't is a contraction of must not, and provided that the use of the contraction doesn't imply some other meaning (which it doesn't) the two must be equivalent, as far as I know.


That is the paradox really - in theory you are perfectly right. In practice, I would still never use the contraction in your example.


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## JustKate

The contraction _mustn't_ just isn't used much these days, at least not in AmE. Yes, they mean the same thing, but to me, _mustn't_ sounds thoroughly Victorian. I would never use it unless I was trying to sound formal and old-fashioned, and come to think of it, that would probably be in a prohibition, such as "You mustn't do that." Otherwise, I would invariably use _must not_.


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## TheKingOfSpain

Seems like we are all in agreement that "mustn't" mustn't be used unless you want to sound old-fashioned and prohibitive. The other is just a style discrepancy, I suppose, although I'm supporting a style I would never use, haha .


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## kalamazoo

Here is my summary.  "Could not" and "couldn't" are basically identical.  They can both express either impossibility or uncertainty and there is no clear distinction. "mustn't" is old fashioned.  "Must not" expresses uncertainty, shading over into impossibility.  As usual, all depends on context.


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## TheKingOfSpain

kalamazoo said:


> Here is my summary.  "Could not" and "couldn't" are basically identical.  They can both express either impossibility or uncertainty and there is no clear distinction. "mustn't" is old fashioned.  "Must not" expresses uncertainty, shading over into impossibility.  As usual, all depends on context.



How can "couldn't (have)" express uncertainty?


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## Forero

The fact is that prohibiting the conclusion that she saw you is, practically speaking, the same as forcing the conclusion that she did not see you, so "mustn't have seen" and "must not have seen" are synonyms. (They must be.)


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## kalamazoo

"I'm sure she couldn't  have seen you, otherwise she would have greeted you."


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## Ariel Knightly

TheKingOfSpain said:


> Well, as a final note, I'm not sure that mustn't has ever been used in a situation like this. It would always be must not in practice. The "more common" usage of mustn't is in phrases like "You mustn't do that". However, the fact remains that mustn't is a contraction of must not, and provided that the use of the contraction doesn't imply some other meaning (which it doesn't) the two must be equivalent, as far as I know.


Yes, but maybe musn't can only be a contraction of must NOT, and not of must _|_not_|,_ which would explain why musn't sound so weird in the context we're talking about.


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## TheKingOfSpain

Ariel Knightly said:


> Yes, but maybe musn't can only be a contraction of must NOT, and not of must _|_not_|,_ which would explain why musn't sound so weird in the context we're talking about.



"Must NOT" and "must |not|" have the same meaning. There is no difference in the meaning of any form of "must not" ever, even though "mustn't" is rarely said.

Replying to kalamazoo:
I don't think that is uncertain. On the contrary, I think that is expressing a certain idea: "She definitely did not see you". In that sentence, the meaning is not changed if you replace could not with must not. "Could" is being used as a past form of can, i.e. "was able to". I think it is also equivalent to say "I'm sure she wasn't able to see you, otherwise...". I see no uncertainty here. Maybe I don't know what is meant by uncertainty?


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## kalamazoo

I think in English,words like "must" and "could" often mean that you are guessing, thus the slight element of uncertainty.  If I knew for 100% sure that she didn't see you, then I probably wouldn't say "I'm sure she couldn't have seen you."  

For example, the following dialogue is perfectly possible:
A.  "Mary walked by me today and didn't even say hello."
B.  "That's not like her.  I'm sure she couldn't have seen you, otherwise she would have said something."
A. "How do you know she didn't see me?  Maybe she's mad at me."
B.  "Well, I don't really know,but I just think that if she had seen you she would have greeted you."


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## TheKingOfSpain

Well, ok. I agree here. You're right about that. I still don't think couldn't and could not are as equivalent as you say in your above post, due to the could |not| case discussed in this thread which I don't think can be meant by couldn't.


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## panjandrum

There is another thread (at least one) about the placing of "not" where something like the sense that King of Spain explains is described.

An important part of the discussion is that the "it's possible that she did not see you" interpretation of "she could not have seen you" is unusual, and is clearly distinguished in spoken English from the "it is not possible that she saw you" version by phrasing and emphasis.

"She COULD not have seen you." - It is possible that she did not see you.
"Not" is part of the clause that is possible.
This form indicates the possibility of something contained in a following clause that is negative.

"She could NOT have seen you." - It is impossible that she saw you.
"Not" is negating the verb "could".
This form indicates the impossibility of something contained in the following clause.

Sorry if this is repeating something in the earlier posts.


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## kalamazoo

I respectfully disagree,although I do understand what you mean.  If you tell me that Mary didn't greet you, how exactly could i ever KNOW for certain that she didn't see you? It's almost a question of my personality and how well I think I know Mary, whether I say "Oh no, [I am certain that] she could NOT have seen you [because I am sure she is not the kind of person not to say hello if she had seen you]" but it still just means that it's my opinion, I don't truly have direct knowledge, and thus it still has a slight element of uncertainty, shading over into impossibility.

The uncertainty in all this doesn't really mean "I have no idea whether she saw you or not" but just "I am pretty sure/very sure/perfectly sure that she must not have seen you."

Of course, probably in a different context, one could say "She could NOT have seen you, because she was 100 miles away at the time."

So I don't think what you are saying is inherently wrong and in fact it's pretty much correct for many situations but I just don't see it as a hard and fast rule either, because it still depends on context.


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## TheKingOfSpain

Sorry, perhaps I've been unclear. The reason I don't think couldn't and could not are equivalent is this example:

A- I'm waiting for a call. I saw Mary across the street today, and I'm pretty sure she saw me. She's going to call to ask what I'm doing here.

B- How do you know she's going to call? She could |not have seen you|. (Alternatively: She could have not seen you.)

This works fine

whereas:


A- I'm waiting for a call. I saw Mary across the street today, and  I'm pretty sure she saw me. She's going to call to ask what I'm doing  here.

B- How do you know she's going to call? She couldn't have seen you.

The couldn't in the second example cannot possibly mean what the could not in the first example meant.

This is all I disagree on. If you do not disagree with this, then we are in full agreement!


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## kalamazoo

Huh?  I don't see any clear distinction.  Either way, it's just B's opinion that Mary may not have seen you, although B might be more or less emphatic or sure about his or her opinion.  How does B know that Mary couldn't have seen you or that Mary could not have seen you?  These are just matters of slight degree.

Either way, A could certainly reply to B:"No, I am pretty sure she saw me. She waved to me!"


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## TheKingOfSpain

Regardless, I think this discussion has strayed far afield. To me the second answer doesn't make sense, because we're assuming here that B was not there, so what he is saying is all conjecture.

Maybe I'm wrong. My brain hurts excessively from doing math. I'm prepared to take it on your authority that I'm wrong, this is just the way it seems to me at this point in time.


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## Ivan_I

TheKingOfSpain said:


> "Couldn't" can only be synonymous with "could NOT".


Really? How come then "can not" and "can't" are not synonymous?

I can't read. (I am unable to read)
I can not read. (I am able not to read.)


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## Myridon

Ivan_I said:


> I can not read. (I am able not to read.)


That is a usage of "not read" not a usage of "can not".


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## Ivan_I

Myridon said:


> That is a usage of "not read" not a usage of "can not".



OK. Then the same should apply to this:

I couldn't read. (I was unable to read)
I could not read. (I was able not to read.)


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## Myridon

Ivan_I said:


> OK. Then the same should apply to this:
> 
> I couldn't read. (I was unable to read)
> I could not read. (I was able not to read.)


I can/could not read. is normally read as "I (can/could not) read."  You are unusually forcing it to be "I can (*not *read)" where "not" is attached to "read" not to "can." If you really mean this, in speech, you would have to stress "not". You would have to use a hyphen or quotes in writing unless the context made it clear.


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## Chasint

Ivan_I said:


> Really? How come then "can not" and "can't" are not synonymous?
> 
> (a) I can't read. (I am unable to read)
> (b) I can not read. (I am able not to read.)



This, and much of this thread, confuses propositional logic with actual usage by native speakers. It is conceivable but very unlikely that (b) would be intended to mean _I am able not to read_.

If you want it to mean that in normal speech, you would have to provide some unlikely and artificial context.

(cross-posted with Myridon - we agree)

_____________________________________________________________________________________

P.S. To be clear, under normal usage, both (a) and (b) have precisely the same meaning, i.e "I am unable to read"


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## Ivan_I

Chasint said:


> If you want it to mean that in normal speech, you would have to provide some unlikely and artificial context.



The degree of unlikeliness is a subjective notion. Have a look at this article. They provide such examples.

Cannot, Can Not or Can’t, Which Should I Use? – Grammarist

But my concern is whether the same can apply to "could not".


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## Myridon

Ivan_I said:


> The degree of unlikeliness is a subjective notion. Have a look at this article. They provide such examples.


An article on grammar is not meant to be "natural".  If you want to use the concept that there is such a thing as "to not-read", then you can use that concept in any way that you can use with reading. 
John can not-read. John is not-reading the paper.  He's holding it in front of him and analyzing the letters in some way that constitutes "not reading".


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## Ivan_I

Myridon said:


> An article on grammar is not meant to be "natural".  If you want to use the concept that there is such a thing as "to not-read", then you can use that concept in any way that you can use with reading.
> John can not-read. John is not-reading the paper.  He's holding it in front of him and analyzing the letters in some way that constitutes "not reading".


I agree. What about "could not"? Can it mean the same as "can not"? I mean ""to not-read" in the past.


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## Myridon

Ivan_I said:


> I agree. What about "could not"? Can it mean the same as "can not"? I mean ""to not-read" in the past.


In your suggested sentence-and-meaning combination, there is no "could not", there is "could" and "not read".  Since "not read" is not a normal usage, no one would read "He could not read" in that way without a very specific context to let them know what "not read" means.


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## Ivan_I

Myridon said:


> In your suggested sentence-and-meaning combination, there is no "could not", there is "could" and "not read".  Since "not read" is not a normal usage, no one would read "He could not read" in that way without a very specific context to let them know what "not read" means.


I think you are being TOO technical. In practical speaking it has no significance how it is called or parsed. But I take your answer as yes.


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## Chasint

Ivan_I said:


> The degree of unlikeliness is a subjective notion. Have a look at this article. They provide such examples.
> 
> Cannot, Can Not or Can’t, Which Should I Use? – Grammarist
> 
> But my concern is whether the same can apply to "could not".



_“Johnny can go to the fair or he can not go to the fair.” _

The above sentence is exactly the artificial and unnatural usage that I'm referring to. The second half only makes the sense you require in the context of the first half. In any natural conversation we would say something like, "_“Johnny can go to the fair or he can stay at home.” _ However even that is unlikely without further context. Why would anyone say this?

Given that the idea is so unlikely for "can", it is even more unlikely for "could".  Please give up this idea. Natural language is not about rules, it is about usage from which approximate rules can be derived. Furthermore, in English, context can entirely reverse the meaning of a sentence under some circumstances.


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## tunaafi

Ivan_I said:


> What about "could not"? Can it mean the same as "can not"? I mean ""to not-read" in the past.


Not in any normal context.


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## Forero

Unfortunately, there is no "couldnot" analogous to "cannot".

The fact is that "could not" can mean "would be unable to" and "would be able not to", as well as just the past tense of both "can not" and "cannot".

And "could not" sometimes means "might not", with all its own ambiguity.

So it takes context to disambiguate "could not", and we are used to living with lots of ambiguity when it comes to "would", etc.

The most common meaning of "could not" is "would not be able to".


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## Chasint

Forero said:


> ...
> 
> The most common meaning of "could not" is "would not be able to".


Even that is debatable. Again only context can tell, but for me it usually means "was not able to".

e.g. "John tried to escape but could not open the door."


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## Ivan_I

Chasint said:


> _“Johnny can go to the fair or he can stay at home.” _ However even that is unlikely without further context. Why would anyone say this?


A very strange stance in my opinion. People can want to convey almost endless nuances of meanings.


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## tunaafi

Right, but in this forum we generally try to present language examples that don't need unusual or extra context.


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## Chasint

Ivan_I said:


> A very strange stance in my opinion. People can want to convey almost endless nuances of meanings.


In English, voice-tone and context convey nuance. In the absence of these, we can only take the 'straight' meaning. Please note the opinions of the native speakers here. Until you accept the advice of people who have spoken the language from childhood, you will not truly understand English.


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## Roxxxannne

I think it's natural and not uncommon in English, at least in American English, to use 'could not + verb,' where 'not' goes with the verb rather than with 'could', when considering a choice:
1) I could take this cough medicine, which has weird side effects, or I could NOT take this cough medicine and just keep coughing.
2) I could re-write my paper for the sixth time, or I could NOT re-write it and give it to my colleague to read instead.

These are different from "was unable to":
a) I could not take the cough medicine without worrying about the side effects.
b) I could not bear to re-write my paper again, so I gave it to my colleague to read.

In cases 1) and 2) one emphasizes 'not' by pausing very slightly before and after it and also by using a different tone or loudness of voice for 'not.'
In cases a) and b) one doesn't use any special vocal emphasis when saying 'not.

In cases 1) and 2), it's obvious from the context (the first half of the sentence) that 'not' goes with  'take' and 're-write.'


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## velisarius

I could use either _can not_ or _could not_ when suggesting an option, with no real difference in meaning. It's the usual difference, with "could" being rather more remote or tentative. 

_- I've been getting strange messages from another forum member. What shall I do? 
- Well, there are several options:* you can/could just not open them, or *you can/could ignore them, or even report them if they're offensive.

- And what if they keep on sending messages? 
- You *can always just not reply, *and hope they get fed up._


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## dojibear

I was a typical American child. When I was around 8 years old, I realized that the sentence "Joe can not read" had two logical meanings:
- Joe is unable to read.
- Joe is able to avoid reading.

I also learned (or I already knew) that this sentence is always used with the "unable" meaning. This is a well-known ambiguity in English. The problem is that "not" is adjacent to 2 verbs (can, read). One common way to remove the ambiguity is to write this:

- Joe cannot read. (Joe is unable to read)

Another way is to learn that "unable" is the standard meaning. When a speaker intends the "avoid" meaning, they use a different sentence.

Often they seperate "not" from "can" while keeping "not" next to "read". For example, post #63 says "can always just not reply". People also say "can choose not to read", "can decide not to read", "can refuse to read", "can avoid reading", and dozens of other things. No-one is required to use an ambiguous sentence, in a language with many alternatives.

In speech, the sentence can also be changed by intonation and pauses, to connect "not read" while disconnecting "can not".


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## Forero

dojibear said:


> I was a typical American child. When I was around 8 years old, I realized that the sentence "Joe can not read" had two logical meanings:
> - Joe is unable to read.
> - Joe is able to avoid reading.


I disagree with this. As I see it, "can't" and "cannot", always pronounced with only one "n" sound, have only the "unable" meaning, but "can not", pronounced as two words and therefore with two "n" sounds (or a double-n sound, if you will), has only the "avoid" meaning.


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