The modal verb must is used for many expressions.

  • She must smile more often. (Suggestion)
  • You must bring along your own calculator. It’s not provided. (Command/Advice)
  • She has been talking for so long her mouth must be very dry now. (Opinion)
  • All of you must come over for the tea party next weekend. (Invitation)
  • You haven’t eaten a pear for a long while, and you think you must pluck one from your neighbour’s tree? (Desire)
  • You have a big pond, so you must keep some ducks. (Insistence)

 Must is used to show something is required.

  • You must sign the visitor book.
  • All applicants must fill in all the pink forms.
  • We must stop at the library to renew the books.

 Must is used to show something is necessary.

  • You must follow the step-by-step instructions to get it working.

  • The advice is she must do 30 minutes of vigorous exercise every day to shed off the excess fat.

  • You must see the doctor about that lump on your cheek.

 Must is used to show something is required by a rule or law.

  • All passengers must gather here fifteen minutes before departure.
  • You must obey the rules and not bring your pet monkey here. 

 Must is used to show something that is very likely or possible.

  • They must be arriving soon.
  • It must be almost one hundred people in that long queue.
  • He must be the skinniest member of his family. 

 Must is used to emphasize a statement.

  • I must say that big man was a real coward. (Emphasis)
  • You must admit you are wrong, and they are right..

 Must is used in questions to express annoyance or anger.

  • Must you visit the toilet so many times?
  • Why must you always say I’m wasting my time watching television?
  • Must you always be as stubborn as a mule? 

 We use had to, not must to express obligation and necessity in the past.

  • Yes: I didn’t pass my exam and had to sit it again.
  • No: I didn’t pass my exam and must sit it again.
  • Yes: Yesterday, we had to take a taxi home when the rain began to fall.
  • No: Yesterday, we must take a taxi home when the rain began to fall.

 Must is used for a future action in the past.

  • It was still a long way to go, but we knew we must arrive in the city before it got dark.
  • The police had no leads in the case, but they must carry on investigating into the woman’s murder.