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

Be able to

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