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?
He should have given us some extra time for the exam.
When do we use would, should, could have been?
One important thing to remember is that modal verbs, in modern English, are tenseless.
We use [modal + have + PP (third verb form)] when we want to give our opinion about a finished action, as in your example,
"He should have given us some extra time for the exam" = In my opinion, he didn't give us enough time for the exam and it would have been a good thing if he had done so.
What's most important to remember is that each modal can have a number of different meanings depending on the situation, even upon the intonation, or the accompanying word choices.
If you have more questions, and I can't imagine that you wouldn't, as the modals are tough to understand, please feel free to ask.