Usually, "can't do" refers to a present event while "can't have done" a past event.
My question is, can "can't have done" refer to a present event? For example,
I worked with him in Shanghai for 10 years,and he _____ now.
A. can't forget me.
B. can't have forget me.

I feel somewhat puzzled here. What about the following sentences?
He can't have finished his homework yesterday.
He can't have finished his homework now.
He can't finish his homewor know.(hmmm, maybe "can't" should be "couldn't" here, but the negative form of can does have the epistemic meaning, doesn't it? )


Many many thanks!

This message has been edited. Last edited by: Mengxin2009,

I'm not sure I understand the intended meaning in your first sentence (given the options you've provided). From a strictly grammatical point of view, A is grammatically possible and B is not.

With A in the blank, the meaning seems to be that he is unable to forget you. Perhaps he has been trying to forget you (and is still trying), but he finds it impossible to forget you.

You might use "can't have forgotten", for example, if someone tells you that he has forgotten you, and you find that hard to believe:

A: He says he doesn't remember you.
B: He can't have forgotten me. I worked with him in Shanghai for 10 years!

In the example above he forgot you at some unspecified time in the past, and he still doesn't remember you. You have just been confronted with this fact, and you firmly believe that it is not possible that this fact is true.

This message has been edited. Last edited by: *Amy*,

quote:

I feel somewhat puzzled here. What about the following sentences?
He can't have finished his homework yesterday.
He can't have finished his homework now.
He can't finish his homework now.(hmmm, maybe "can't" should be "couldn't" here, but the negative form of can does have the epistemic meaning, doesn't it? )

- He can't have finished his homework yesterday. = I do not believe it is true that he finished his homework yesterday.

- He can't have finished his homework now. This sentence doesn't really make sense, but it would be OK if you changed the word "now" to "already":
- He can't have finished his homework already. = I have reason to believe that it is not true that he finished his homework at an unspecified time before now.

- He can't finish his homework now. = It is not possible for him to do his homework now. He has not finished his homework, and it is not possible for him to finish it now/in the future.

This message has been edited. Last edited by: *Amy*,

"Can't have done something" means that it was absolutely impossible to have done something. It is even stronger than 'could not have done something" or 'must not have done something."

For example, let's say that there was a murder over the weekend, the murder of Mr. Smith, a banker, by gunshot in his office in New York.

1) Did Jack commit the murder? No, Jack {must not have committed the murder. He was a good friend of Mr. Smith. (95% sure.)

2) Did Bill commit the murder? No, Bill couldn't have committed the murder. Bill doesn't own a gun, and doesn't like guns. Besides, Bill is a calm person who never gets angry. (97% sure.)

3) Did Dick commit the murder? No, Dick can't have committed the murder! Impossible!Dick was in Antarctica last week, and he's still there. I got a phone call from him on his ship this morning. (100% sure.)