can we equally use can't/couldn't to say something is not possible?
Such as in:
It is too early, it can't/couldn't be the postman yet.
It was too early, it can't/couldn't have been the postman yet.
Thank you for diffusing my doubt about it and your time.
NOT A TEACHER
(1) It may/might not rain tomorrow. = Perhaps it will not rain.
(2) It can't/couldn't possibly rain tomorrow. = It certainly will
Source: Mr. Michael Swan, Practical English Usage (1995 edition,
item #123.3 -- not page number).