Must remains unchanged whatever be the tense or the number and person of the subject. It can refer to the present or future. It can point to the past only when it is used with the present perfect tense of the principal verb.
Compare:
- He must go home. (Future / present)
- He must have gone home. (Past)
- We must see the minister now. (Present)
- He must have seen the minister by now. (Past)
- You must file a writ petition. (Future)
Uses of must
Must expresses compulsion or strong obligation. It is much stronger than should.
- He must apologize for his mistakes.
- They must pay the fine.
- You must be loyal to your country.
Necessity
- We must get up early and start on our way.
- Must we wait for them?
Probability or likelihood
Must can express probability or likelihood.
- He must be mad to do this.
- Oh, there is the door bell; that must be the postman.
Strong determination
Must signifies strong determination.
- I must have my own way.
Ought
Ought is different from other modal auxiliary verbs; it is followed by an infinitive with to.
Compare:
- I ought to go.
- I must go.
- She must wait.
- She ought to wait.
Ought expresses ideas such as duty, necessity, moral obligation etc. It is not as forceful as must, but it is stronger than should.
- We ought to help the poor and the needy. (Duty)
- You ought to exercise regularly. (Necessity)
- She ought to be back by 10 o’clock.
When ought refers to past time, it is followed by the perfect infinitive.
- You ought to have helped him.