Must - English Grammar


English Grammar - Modal Verbs

Must is a modal verb

Let's look at the different uses of MUST:


1. To express obligation or duty

This also refers to laws and regulations.

2. To emphasize the necessity of something

3. Deduction - Sure that something is true (Certainty)

We use this when we don't know but we are certain that it is true (based on evidence).

4. Expresses positive logical assumptions (Must + have + past participle)

5. A strong recommendation

Something that is highly recommended (stronger than using should)

Must Summary Chart

The modal verb MUST in English


The negative is Mustn't which refers to prohibition (negative obligation)

Mustn't = Must not

Must vs. Have to

Must can be replaced by Have to with little difference in meaning:

Have to is a more informal while Must is mostly used in written orders or instructions.

Also, Must expresses obligation imposed by the speaker while Have to expresses external obligation.

When we are mentioning someone else's obligations, we use Have to.

For questions it is more common to use Have to instead of Must (which sounds very formal):

The past tense of Must is Had to:

Mustn't vs. Don't have to

Be careful with the negative of Must and Have to where they DO have a different meaning. Mustn't is a negative obligation (= it is important that you do NOT do something) while Don't have to is an absence of obligation.

Mustn't = it is prohibited; it is not allowed
Don't have to = no obligation; you are not required to do something, especially if you don't want to.

The difference between MUSTN'T and DON'T HAVE TO in English

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See our notes about other Modal Verbs.

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