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!