question(grammer)

[sandykhattar]

Hi,

I have two questions about the grammer.I always forget this rule . What is the best way to remember.

When do we use would, should and could plus verb third form?
For example
He should have given us some extra time for the exam.

When do we use would, should, could have been?


Thanks

[Tdol]

He should have given... This implies that he was in some way wrong not to give us the time.
He could have... It was possible for him to do it, but he didn't
He would have given... Something stopped him from doing it