In the following sentence, to talk about the present possibility can I use ' can' ? If so, what is the differnce between can and could?

Who stole the money?

It can be John.

It could be John.

It can't be John.

It couldn't be John.
In my opinion 'It could be John' is contextually correct.

'Can' is present and 'could' is past.

John's act is based on the possibility in the past
Who stole the money?
It can be John. << not used
It could be John. << = It may/might be John = It is possible that John is the one (who stole the money).
It can't be John. << = It is not possible that John is the one (who stole the money).
It couldn't be John. << = It would not be possible for it to be John.

CJ
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Hi CalifJim

Can we use "It can't be John" to refer to something in the past.
vsureshCan we use "It can't be John" to refer to something in the past.
No, not in the general case. Use "It couldn't have been John" for the past. It means "It is not possible that it was John".

Oh, I see what you mean. "It can't be John" is not really a reference to the past in the case we discussed above. It's a matter of the current (present) identity of the thief. Here we're talking about the identity of someone, which remains the same now as it was when the money was stolen.

Here's a different way of phrasing it.

Who stole the money?

It can't be John. = It is not possible that John is the one who stole the money.

It couldn't have been John. = It is not possible that John was the one who stole the money.

These are effectively the same in meaning, but with a different focus with regard to time.

CJ

This is a better phrasing, so I'm going to change the phrasing in my previous answer.
Your explanation is very clear. I am on the ball.

Thank you, CalifJim.
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I am still confused between the following two sentences.

It couldn't be John.

It can't be John.

When I say ' It couldn't be John', does it imply that there is still possibility it is John who stole the money? but

If I say, It can't be John, does it mean that there is no possibility for him to have stolen the money?
Great Answer
sun 94When I say ' It couldn't be John', does it imply that there is still possibility it is John who stole the money? but
If I say, It can't be John, does it mean that there is no possibility for him to have stolen the money?
No matter what you say, there's always the possibility that you could be wrong.

For all practical purposes, the two mean the same thing.

It can't be John. - stronger statement - The speaker feels he can't be wrong. Maybe he's an eyewitness.

It couldn't be John. - weaker statement - Maybe the speaker doesn't have solid evidence, so he is basing his statement on other factors.

CJ
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