modals. Degrees of certanly

Certainty (or near certainty) about now, the future or generally

will, would, must, can, can’t, could, couldn’t

There’s someone at the door. That will be the postman.

Certainty (or near certainty) about the past

will have, won’t have, would have, wouldn’t have, must have, can’t have, could not have

They won’t have heard the news, will they?

They must have heard by now, surely.

Probability about now, the future or generally

Should, shouldn’t, ought to, oughtn’t to, may/might well, could well, might easily

The weather should be good tomorrow, shouldn’t it? Actually, the forecast said it may well rain.

Probability about the past

Should have, shouldn’t have, ought to have, oughtn’t to have, may/might well have, might easily have

Jan should have finished writing her article by now, shouldn’t she? She may well have done, but i haven’t seen it yet.

Possibility about now, the future or generally

could, may, might, mightn’t, may/might/could just

I might (just) have time to get to the library before it closes.

Possibility about the real past

could have, may(not) have, might have, mightn’t have

Jim might not have checked his email yet.