Must and Ought to

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 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.)