Must and Ought to

August 16, 2010pdf

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.)

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