Be able to
Be able to can be used to talk about ability. It often has the same meaning as can/could.
- He is able to support her. (He can support her.)
- They were able to catch the thief. (They could catch the thief.)
Be + infinitive
Be + infinitive is used to indicate simple future, like shall or will, but perhaps with a slight degree of uncertainty.
- He is to retire. (It is not certain whether he will actually retire or not.)
- We are to have a test in English next week. (It is not certain whether the test will actually take place.)
Be + infinitive can also have a similar meaning to must.
- You are to complete the work in two days. (You must complete the work in two days.)
- He is to report for duty within a week. (He must report for duty within a week.)
Had better can have the same meaning as should or ought.
- You had better get some rest. (= You should/ought to get some rest.)
- We had better consult a doctor. (= We should/ought to consult a doctor.)
Have to and have got to
- I have to be there by 10 o’clock. (I must be there by 10 o’clock.)
- Did you have to do that? (Was that necessary?)
- I have got to see her.