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Hellow teachers, I'm not quite clear about the usage of the word can, it has the meaning of one's ability to do something, e.g. I can see her sitting there in the bar; and it also means there're chances/possibities that some events may happen, e.g. That couldn't be her, she is still on her vacation in L.A.

Here's the question, is it okay to say this? She is still on her vacation in L.A., you couldn't see her at the bar.
Apprently the sentence means that it's impossible for you to see her at the bar because she is on her vacation in L.A.
Some may say it this way, She is still on her vacation in L.A. you can't have seen her at the bar. Is this correct?
I'm not sure if my conclusion is right, my view is that Can or Could would only have the meaning of chances or possibilities when it's followed by the link verb be. Am I right?

All advices are welcomed, thanks a lot!
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Comments  (Page 2) 
It's my fault. The statement "We can’t use the Past Tense verb form to talk about whether something possibly happened or not!" is not clear enough. Hopefully, I can make this clearer:

What about this?~ That couldn't be true! Can we indicate that something was not possible to be true or something was not possibly true by saying this?
Look at these sentences:
A: I heard that Sue and Jim have split up.
B: That can't be true. They booked to go on a holiday together just this morning. (= here and NOW, I say to you that I doubt that this is true - Present Tense)
compare
Z: John told me that Sue and Jim have split up, but I told him that couldn't be true, because...(Past Tense)
compare, from my last post:
These are lousy seats. I can't see anything. (Present Tense)
We went to a rock concert last night. I couldn’t see the performers for all the fans standing and waving their arms in the air. (Past Tense)

Let's go back to your original scenario, but change 'saw' to 'bumped into' (=met by chance) AND withhold the information that your logical deduction is based on:
A: I bumped into Mary in a bar last night.
B: You couldn't bump into her there.
YES YOU CAN! As soon as she's back in town, pop into the bar occasionally, because she often frequents that bar. THEN, it is quite likely you will bump into her! The statement has the meaning, that bar is not a place that Mary goes to!

What you are wanting to say is that last night, the 'bumping into her' didn't happen at all. You then reveal your information - she's out of town - to demonstrate why what he is telling you about is 'unreal', 'must be a figment of his imagination' .........or he's lying; and you could say:
A: I met Mary in a bar last night.
B: That can't be true - you're lying. Mary is out of town.
Here, you are challenging, right then and there, the NOW, the truth of his assertion; whereas in the original, you are saying you cannot accept the situation last night as being REAL.

Does that help? Emotion: smile
Addendum to:
A: I met Mary in a bar last night.
B: That can't be true - you're lying. Mary is out of town.

In the Past Tense that would be:
John told me he met Mary in a bar last night. I all but called him a liar to his face. I told him that couldn't be true, because Mary is out of town.

And further:
I can't run one kilometer, never mind run a 5 kilometer marathon. (Present Tense fact)

I went jogging yesterday, and couldn't even run one kilometer.(Past Tense fact)

John completed the marathon? I don't believe it. He couldn't have run the whole five kilometers (=talking about the UNREAL situation of his actually, actually, running that distance). Last time I spoke to him, he couldn't even run more than one kilometer.(talking about a REAL situation.)
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And one last point:
When you challenge the truth of a statement, you are simply saying, what you say is not true.
The 'Mary in the bar last night' scenario presents you with the unreal situation that Mary has been in two places at once!
You want to respond, not by challenging his veracity, but by talking about the unreality of that situation; and why it is unreal: you say she was in the bar, I believe she was out of town - so if what I believe is true, and she was out of town, 'then she couldn't have been in the bar'.
I've suddenly realized what may be contributing to the confusion.
With your intended response to the 'Mary in the bar' scenario, you are not talking about the truth of what HE has said, but talking about MARY...and her capacity to be both in the bar last night, (so as to be seen by him), and at the same time in LA. I have been so busy looking at 'his seeing/his bumping into" that I lost sight of whom we are talking about:
"You couldn't (something) HER. SHE is in LA."

I think that does it. I think that's the best my neurones can come up with.
RobinGYou can't see her at the bar. ~ It [is / will be] (physically) impossible to find her at the bar.(We can assume that she is blocked from you by a pillar or something, right?)
I take "see" as "find". Being blocked by a pillar is not what I had in mind. What I mean is that you can wander all around the bar looking for her (a physical action), but you will never her find her there because she is not (physically) there. She is (physically) in LA.
RobinGWhile in this one: You couldn't see her at the bar. ~ It was impossible for you to see her at the bar. / You failed to see her at the bar.
So suppose the sentence is correct, let's put it in here, I couldn't find her at the bar yesterday because she is still out of town on vacation.~It was impossible for me to find her at the bar yesterday because she is out of town on vacation. Can I understand it this way?
That's exactly how it works. Yes. I couldn't find her there (It was impossible for me to find her there / I failed to find her there) because she was not there! She was somewhere else.
RobinGThought I might just as well buy a Webster and plough through it all.
While you're at it, get The English Verb by Palmer. It's a fairly short book, but it will have a lot more about can and could (and may and must and all those) than a dictionary. Emotion: smile

CJ
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Thanks Terry it seems getting way too complicated than expected in the first place, guess I need to take some time to contemplate. Really appreciate your help!
Hi CJ, thanks for clearing that up. I have to say it's sort of much too imaginary, have to use my whole imagination to put myself into the assumed scenario.....Emotion: thinking
Looked up The English Verb by Palmer on the internet, cannot seem to find or buy one in China, thanks anyway!
RobinG I have to say it's sort of much too imaginary, have to use my whole imagination to put myself into the assumed scenario...
Yes, it does take some effort to imagine these things, especially at first, but in time you will get more experience and everything will seem easier. Emotion: smile
RobinGLooked up The English Verb by Palmer on the internet, cannot seem to find or buy one in China
Hmm. I'm surprised to hear it. Bad news. Emotion: sad

CJ
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