When to use Must and mustn’t?

When and how to use Must and mustn’t?




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Must is a modal. It exists only in the present tense and always expresses the point of view of the enunciator.

Must allows you to express the obligation (or constraint), a recommendation, but also the probability (or certainty).

1 – Obligation and constraint

You use must to put pressure on yourself or someone else by expressing:

An obligation or constraint with must:

  • You must be back before ten.
  • He must be careful.
  • I must hurry.

Must may also make a recommendation:

  • You must visit the Colosseum when you will go to Roma.
  • You must see this movie ! 

In the negative form, an interdiction with must not (or mustn’t).

  • You mustn’t show it to anybody.
  • She must not climb on the table.
  • We mustn’t wear a cap in the classroom.

2 – Probability and certainty

Must also express a high probability or certainty. It is used when you are almost sure of what you are saying:

  • She must be Japanese.
  • He must be tired. 
  • You must be John’s brother. 
  • There must be some mistake. 

In the negative form, to say that something is impossible, we use can’t (and not mustn’t):

  • He can’t be Spanish.
  • They can’t be sleeping, it’s already noon.

To express the probability/certainty in the interrogative form, we use can (not must):

  • Can it be false? 

To express certainty or probability in the past tense, we use must + have + verb in the past participle:

  • I must have left my phone at home.
  • Paul must have seen this movie before.
  • It must have been raining last night.
  • He must have missed the train.
  • Someone must have taken my umbrella by mistake.

Be careful not to confuse the two possible meanings of must + have + verb in the past participle: either obligation or probability:

  • Paul must have learned this lesson by monday  (obligation)
  • Paul must have learned this lesson before  (probability)

In the future tense, we use ‘have to‘:

  • When you’ll be in Paris you will have to see the Eiffel tower.

3 – Must or Have to ?

You will find the complete lesson on the difference between must and Have to in this lesson!



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