Modal verbs in English – Must and Must not

Must and Must not is a popular keyword and we did our best to create this YT video around this keyword

When to use Must

  • We use must: to express the idea that something is necessary or very important.
  • With must, the speaker is expressing a personal opinion or authority.
  • Must is used in written instructions and orders.
  • We also use must in: Obligation, Prohibition, Strong advice, and Deductions.

When to use Must not

Must not = mustn’t.
Mustn’t is a negative obligation, it is important that you don’t do something.

Must and Must not Form

Subject + must or must not + base verb + complement.

The main verb is used without, “to”, “s”, and ing.

For example:

  • He must work hard to pass. (correct)
  • He must works hard to pass. (incorrect)
  • We must go home. (correct)
  • We must to go home. (incorrect)
  • You must respect your parents. (correct)
  • You must respecting your parents. (incorrect)


Don’t use must or must not in the Past.
We use had to in the Past.

Examples of Had to

  • I was very hungry yesterday, I had to eat something.
  • We had to walk home last night, there were no buses.

Must and Must not Examples

  • You must not smoke.
  • You must not drink wine.
  • It’s late, we must go now.
  • We mustn’t shout in class.
  • We mustn’t miss our plane.
  • We mustn’t cheat in an exam.
  • We mustn’t park in this street.
  • I mustn’t forget to call my wife.
  • We must finish this work today.
  • You must finish your homework.
  • I must apologize for arriving late.
  • You must not be late for the exam.
  • It’s a fantastic film, you must see it.
  • We mustn’t leave the meeting early.
  • I’m very hungry. I must eat something.
  • My hands are dirty, I must wash them.
  • Students must not talk during the exam.
  • Plants must have light and water to grow.
  • We must respect our parents and teachers.
  • Your wife must serve you and your children.
  • We must not be late, we must come on time.
  • You must take this medicine three times a day.
  • I must get up early tomorrow, I have a lot to do.
  • You must watch this lesson, it’s really important.
  • The windows are very dirty, we must clean them.
  • You must not use your phone while you are driving.
  • I mustn’t forget to tell John, he wants to come, too.
  • You mustn’t bring your mobile phone into the exam.
  • We mustn’t lose our tickets, we can’t get any more.
  • Sally is a very interesting person, you must meet her.
  • Keep these papers in a safe place, you mustn’t loss them.
  • We must not forget to turn off the lights before we leave.
  • You mustn’t throw things in the science lab! It’s dangerous!
  • You mustn’t get the number 6 bus, it doesn’t stop at the station.
  • I forgot to phone David yesterday, I must phone him later today.
  • You must study before the exam if you want to get a good result.
  • I don’t want Susan to know what happened, you mustn’t tell her.
  • We must check the times of the trains again. I’m not sure of them.
  • It’s my daughter’s birthday next week, I must not forget to buy her a present.

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