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Strong obligation: must, will

The following explanations apply particularly to British English. Americans often use have (got) to where British people use must (see here). However, this use of have (got) to is becoming more common in British English under American influence. And note that must is becoming less common in both British and American English.

Statements: I really must stop smoking.​

In affirmative statements, we can use must to say what is necessary, and to give strong advice and orders to ourselves or other people.

  • Plants must get enough light and water if they are to grow well.
  • British industry must improve its productivity.
  • I really must stop smoking.
  • You must be here before eight o’clock tomorrow.

Must is common in emphatic invitations.

  • You really must come and see us soon.

Questions: Must I …?​

In questions, we use must to ask about what the hearer thinks is necessary.

  • Must I clean all the rooms?
  • Why must you always bang the door?

Negatives: You mustn’t/can’t …​

We use must not / mustn’t to say that it is wrong to do things, or to tell people not to do things. Can’t is also possible.

  • The government mustn’t/can’t expect people to work for no money.
  • You mustn’t/can’t open this parcel until Christmas Day.

mustn’t and needn’t / don’t have to​

Mustn’t is not used to say that things are unnecessary. This idea is expressed by needn’t (BrE), don’t need to (see here) or don’t have to.

  • You needn’t work tomorrow if you don’t want to. (BrE) or You don’t have to work … (not You mustn’t … if you don’t want to.)

  • You don’t need to get a visa to go to Scotland. or You don’t have to get a visa … (not You mustn’t get a visa to go to Scotland.)

Past necessity and obligation: They had to work very hard.​

Must is not normally used to talk about past obligation (except in indirect speech – see below). This is because must is used mainly to influence people’s behaviour – for example through orders or advice – and one cannot do this in the past. Had to is used to talk about obligation that existed in the past.

  • I had to cycle three miles to school when I was a child.
  • My parents had to work very hard to build up their business.

Note the difference between had to + infinitive and must have + past participle. The structure with must have … is used to express certainty about the past (see here). Compare:

  • Nadia isn’t in her office. She had to go home. (= It was necessary for her to go home.)
  • Nadia isn’t in her office. She must have gone home. (= It seems certain that she has gone home.)

Orders with will: Will you be quiet!​

We use will you to tell people what to do.

  • Will you send me the bill, please?
  • Come this way, will you?
  • Will you be quiet!

Will can be used in affirmative structures to give impersonal, military-type orders.

  • All staff will submit weekly progress reports.

Indirect speech: He said I must stop.​

Must can be used after a past reporting verb as if it were a past tense.

  • The doctor said that I must stop smoking.

Obligation can also be reported with had to and would have to.

  • The doctor said that I had to / would have to stop smoking.

must and should​

Should can be used as a weaker form of must (see here). Compare:

  • That carpet must be cleaned. (= It is absolutely necessary.)
  • That carpet should be cleaned. (= It would be a good idea.)