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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.

  • must memorize all of these rules about modal verbs.
  • People must remain seated until the show is over.
  • You must wear a seatbelt at all times.

2. To emphasize the necessity of something

  • Humans must have drinking water at least every two days.
  • You must give up smoking, it's bad for you.
  • We must have a special permit to camp in the national park.
  • You must study the last two chapters before the test.
  • Plants must have light and water to grow.
  • You must drive carefully.

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).

  • Look at all of that snow. It must be really cold outside.
  • The ground was wet this morning. It must have rained last night.
  • Dinosaurs were very big, they must have eaten a lot.
  • It's five in the morning and you still haven't gone to bed? You must be tired!
  • Jack must be home. I heard a noise coming from his room.

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

  • That must have been my mother calling me last night, nobody else has my number.
  • He must have won the lottery with the new house and car he has just bought.
  • She must have been at home - her car was there.

5. A strong recommendation

Something that is highly recommended (stronger than using should)

  • We really must get together for dinner sometime.
  • You must see the new Peter Jackson movie, it's fantastic.
  • The ice cream here is delicious. You must try some.

Must Summary Chart


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

Mustn't = Must not

  • You mustn't use your smartphone while you are driving.
  • You mustn't get on the subway if you haven't paid for the ride.
  • You must not open the gift until it is your birthday.
  • We must not tell anyone.

Must vs. Have to

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

  • You have to study. (= you must study)
  • He has to finish the report by Friday. (= He must finish the report by Friday)
  • They have to resit the test. (= They must resit the test)

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.

  • Teacher: You must complete this essay by Friday
  • Student: We have to complete this essay by Friday.

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

  • John has to quit smoking.

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

  • When do you have to pay finish the report?
  • Does he have to take a blood test?

The past tense of Must is Had to:

  • I had to pay my speeding ticket yesterday.

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.

  • You must not drink that. (= it is forbidden to drink that; it is not allowed)
  • You don't have to drink that. (= you don't need to drink that but you can if you want)
  • You mustn't tell John (= Do not tell John)
  • You don't have to tell John (= you can tell John if you want to but it is not necessary)

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