Can’t ( or couldn’t) have + past participle. : This expression is used to explain the opposite of ‘Must Have’ when it deals with the probability aspect. Examples will clarify the essence of this expression.
- Johny drove past me without stopping. He can’t have seen me.( i.e probably he would not have seen me. We don’t say, he mustn’t have seen me) . Now differentiate the expression ‘must have ‘ when used in a positive sentence. Johny stopped his car on the way. He must have seen me walking and would have liked to offer me a lift.
- ‘ He couldn’t have seen me. ‘ can be used in place of ‘ He can’t have seen me ‘.
- The drunkard walked straight into a wall. He can’t have seen the wall.
- He couldn’t (or can’t) have met the minister yesterday. (maybe, the minister was very busy or otherwise engaged)
‘ Can ‘ with different degree of sharpness when used in the negative.
- We are unable to help you. ( the polite and preferred form of denial)
- We cannot help you. (more abrupt form of denial)
- We can’t help you.( the most abrupt form of denial)
Note : ‘ We are unable to ‘ is often preferred in formal English to the more abrupt ‘ cannot’ and the still more abrupt ‘ can’t ‘.