Must

Must: forms

Affirmative (+) form

Must comes first in the verb phrase (after the subject and before another verb):

• She must have lots of friends.

Must can’t be used with another modal verb.

• This must be your sister.
• Not: This must can be your sister. or This can must be your sister.

Negative (−) form

The negative form of must is mustn’t. We don’t use don’t/doesn’t/didn’t with must:

• There mustn’t be any rubbish left.
• Not: There dosen’t must be any rubbish left.

We can use the full form must not in formal contexts or when we want to emphasise something:

• You must not leave any rubbish.

See also

• Modal verbs

Question (?) form

warning

The subject and must change position to form questions. We don’t use do/does/did:

• Must you make that noise?
• Not: Do you must make that noise?

We can use must and mustn’t in question tags though tags with must aren’t very common:

• The house must be worth millions, mustn’t it?

Must: uses

Deductions and conclusions

When we think carefully about facts, we often use must to express deductions and conclusions from these:

• [fact] He’s so small. [deduction/conclusion] He must be no more than four years old.
• [Two teachers talking about a student]
• A: He falls asleep in class every morning. (fact)
• B: He must be out late every night or maybe he works at night. (deduction/conclusion)

warning

We use can’t/cannot as the negative of must to deny something or make negative deductions or conclusions:

• It just can’t be true. He can’t have left his job.
• That cannot be his sister. She looks so different.

See also

• Can

warning

We use must have + ed form and can’t have + ed form to talk about deductions in the past. They always refer to deduction, not obligation:

• [A wanted to talk to B so she phoned him but he didn’t answer the phone. She phoned again the next day]
• A: I called you yesterday around three o’clock but you must have been out.
• B: We must have been in the garden. That’s a pity.
• [A is telling B about his illness]
• A: I spent a month in hospital before I was able to walk.
• B: That can’t have been easy for you.

Spoken English:

In speaking, we very often express our reaction to what we hear using phrases such as that must be or that must have been:

• A: She lives in Thailand now.
• B: That must be amazing!
• A: Twelve years ago Kevin and I went on a six-week camping trip.
• B: That must have been fun.

Obligation and necessity

Must expresses strong obligation and necessity:

• I must talk to you about the new project.
• Seat belts must be worn even in the back of the car.
• There must be a minimum of two members of the company at the meeting.

warning

We use had to not must to expresses obligation and necessity in the past:

• By the time we got back to our bikes, it was dark and we had to cycle home in the dark without any lights …

• Not: … it was dark and we must cycle home in the dark …

• Last year, teachers had to make a report on each child every week.

• Not: Last year, teachers must make a report …

We use must to talk about the future in the past when we report speech or people’s thoughts in formal contexts:

• [Extract from a novel]
• The pain was back in full force, but she knew she must not give in to it. She must go on day by day.

We use will have to more than must to express future obligation, especially when talking about obligations at a particular point in the future:

• He*’ll have towait five weeks for his eye operation. Then he’ll have to** have both eyes operated on.*

We often use must with more general references to the future, particularly when talking about obligations that come from the speaker:

• The Prime Minister must decide in the next month.
• I must try harder next time.
• I must pop round one evening next week.

When we talk about no obligation, we use either need not, don’t/doesn’t/didn’t have to or the negative of the main verb need (don’t/doesn’t need):

• You needn’t worry about it. I’ll take care of it.
• You don’t have to worry about it. I’ll take care of it.
• You don’t need to worry about it. I’ll take care of it.

Rules and laws

We use must not to talk about what is not permitted:

• You must not park outside the entrance.
• You must not make noise after 9 o’clock.

Must and must not often occur in public signs and notices indicating laws, rules and prohibitions:

• [airline website information]
• All passengers must present valid photo identification at check-in for all flights.
• [bus company website notice]
• Tickets must be retained for inspection, and must be produced for inspection on request by any authorised official of Bus Éireann. (Bus Éireann is the name of the Irish national bus company)

See also

• Commands and instructions

Invitations and encouragement

We also use must to express polite invitations or encouragement:

• You must come and see us soon.
• You must try some of this chocolate cake. It’s delicious.
• You must go and see that film.

See also

• Invitations

Criticisms

We use the question form of must in criticisms:

• Must you keep playing that terrible music?
• Why must you mispronounce my name every time?

Must and have (got) to?

Obligations

We usually use must to talk about obligations which come from the speaker and we generally use have (got) to when we refer to obligations that come from outside the speaker.

Compare

I must buy some new clothes. Mine look so old.	The obligation is from me to buy new clothes.
I’ve got to buy some new clothes. I’m starting a new job as a teacher and we have to wear formal clothes.	The obligation is from the school to buy new clothes.

warning

Must not and don’t have to/haven’t got to have different meanings.

Compare

You must not give my credit card details to anyone.You mustn’t tell this to anyone. It’s a secret.	We use mustn’t to talk about something which is forbidden.
You don’t have to tell anyone. I will email everyone.We haven’t got to wear a uniform to work.	We use don’t have to/haven’t got to when something is not necessary. It is not forbidden.

Don’t have to can sometimes be used to criticise someone or to tell them not to do something. This is less direct than must not:

• You don’t have to drink all of the juice! (stop drinking the juice!)

Deductions

We can also use have got to when we make deductions or draw conclusions. Must is more common than have (got) to in this meaning:

• That must be a fake!
• That picture has got to be a fake!

See also

• Have got to and have to

Must: typical errors

We don’t use must to expresses obligation and necessity in the past. We use had to instead:

• When she got home, she had to cook dinner before everyone arrived.
• Not: When she got home, she must cook dinner before …

We don’t use must to make predictions about the future. We use will instead:

• Don’t worry about our accommodation because I found a nice hotel which will be suitable for us.
• Not: Don’t worry about our accommodation because I found a nice hotel which must be suitable for us.

See also

• Will