Must doesn’t change its form, whatever be its tense or the number and person of its subject. It can refer to the present or future.
- You must do this now. (Present)
- He must pay damages. (Future)
- You must file a petition. (Future)
Must can refer to the past only when it is used with the present perfect of the main verb.
- She must have gone home. (Here must refers to the past time because it is used with the present perfect of the verb go.)
- She must have reached home. (Past)
Uses of must
Must is used to express ideas such as compulsion, obligation or duty. It is much stronger than should.
- We must love our country.
- They must recognize our rights.
- He must pay the fine.
Must can be used to talk about necessity.
- We must get up early.
- I must improve my writing skills.
- Must we go now?
Must can express probability or logical certainty.
- She must have already left.
- He must be mad to do this.
- Oh, there is the door bell; that must be the postman.
To signify strong determination
- I must go now, whatever happens.
Ought
Ought is different from other auxiliary verbs: it is followed by a to-infinitive.
Uses of ought
Ought expresses ideas such as duty, necessity and moral obligation. It is not as forceful as must, but it is stronger than should.
- You ought to be punctual.
- We ought to help the poor.
- You ought to visit your friends once in a while.
Ought generally points to present and future time. It can point to past time when it is followed by the perfect infinitive (have + past participle).
You ought to have helped him. (It was your duty to help him but you didn’t.)