These constructions are not grammatically the same as modal verbs (Unit 27), but they have a modal meaning. We always use the base form of the verb (Unit 14) after them.

Be able to

We use it to talk about ability. It is more polite than can (Unit 28) or could (Unit 29).

Will you be able to attend the class today?
I'm afraid I may not be able to attend.
Unfortunately, I wasn't able to attend the class yesterday.

Ought to

We use it for advice, necessity and probability (exactly the same way as should in Unit 33).

You ought not to take it without asking.
I think we ought to have apologized.
It ought to be on the top shelf.

Have got to

We use it to talk about necessity for present or future time (the same as have to in Unit 34).

Have you got to work tomorrow?
I've got to finish this report before I go home.

Had better

We use it for advice and necessity for a particular present or future time. It is stronger than ought to (above) or should (Unit 33).

I think you'd better get some new clothes for the interview.
We'd better be quick or we'll miss the bus.
You'd better not be late again or I'll be very angry.

NOTICE: We nearly always use the short form of had.

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Exercise 36.1
Exercise 36.2