can+perfect infinitive

[ostap77]

I came across a grammatical construction in books that I have not heard in conversation before. I refered to the grammar text-books I have but didn't get a clear answer. There was a short passage didn't completly cover it.

"He can't have said it."

OR

"Can he have done that?" It would mean "I don't believe you. He didn't do it."?

Is it used in conditionals? Do we use it in an effermative sentence?

 

[5jj]

I came across a grammatical construction in books that I have not heard in conversation before. I referred to the grammar text-books I have but didn't get a clear answer. There was a short passage didn't completely cover it.

"He can't have said it."

OR

"Can he have done that?" It would mean "I don't believe you. He didn't do it."?

Is it used in conditionals? Do we use it in an effermative sentences?

Must is used, meaning something along the lines of 'it is logically probable that', mainly in affirmative sentences. Can't is used as its negative equivalent. Neither is normally used in questions.

He must be rich; He must have been rich.
He can't be rich; He can't have been rich.

When we are talking about present or future probable situations, must and can't are not normally used with verbs in a dynamic sense, except in progressive forms:

He must be rich - logical probability.
He must work hard* - obligation.
He can't be rich - logical probability.
He can't work* - ability.
He must be working hard. logical probability.

* In the spoken language, such utterances can be heard with the logically probable meaning when the context and intonation make it clear what is meant.