must

Listen:
UK:*UK and possibly other pronunciationsUK and possibly other pronunciations strong: /ˈmʌst/, weak: /məst/US:USA pronunciation: IPA and respellingUSA pronuncation: IPA/mʌst/ ,USA pronunciation: respelling(must)


WordReference Collins English Usage © 2020
must
Must is usually used to say that something is necessary. It can also be used to say that you believe that something is true. Must is called a ‘modal’.
Modals
‘must’, ‘have to’, ‘have got to’, and ‘need to’
The expressions have to, have got to, and need to can sometimes be used with the same meaning as must.
The negative form of must is must not or mustn't. The negative forms of have to and have got to are don't have to and haven't got to. The negative form of need to is need not, needn't or don't need to. However, these negative forms do not all have the same meaning. This is explained below under negative necessity.
necessity in the present
Must, have to, have got to, and need to are all used to say that it is necessary that something is done.
I must go now.
You have to find a solution.
We've got to get up early tomorrow.
A few things need to be done before we can leave.
After must you use an infinitive without to. Don't use a to-infinitive. Don't say, for example, ‘I must to go now.’
If someone is required to do something regularly, for example as a job or duty, say that they have to do it. Don't use ‘must’.
She has to do all the cooking and cleaning.
We always have to write to our grandparents to thank them for our birthday gifts.
If someone is required to do something on a particular occasion, say that they have got to do it or, in formal English and American English, that they have to do it.
I've got to go and see the headmaster.
We have to take all these boxes upstairs.
In formal English, must is used to say that someone is required to do something by a rule or law.
You must submit your application by the end of this month.
necessity in the past
If you want to say that something was necessary in the past, you use had to. Don't use ‘must’.
She couldn't stay because she had to go to work.
We had to sit in silence.
necessity in the future
If you want to say that something will be necessary in the future, you use will have to.
He'll have to go to hospital.
We will have to finish this tomorrow.
negative necessity
You use must not or mustn't to say that it is important that something is not done.
You must not be late.
We mustn't forget the tickets.
If you want to say that it is not necessary that something is done, you use don't have to, haven't got to, needn't, or don't need to.
You don't have to eat everything on your plate.
I haven't got to work tomorrow, so I can sleep late.
You don't need to explain.
Be careful
Don't use ‘must not’, ‘mustn’t', or ‘have not to’ to say that it is not necessary that something is done. Don't say, for example ‘You mustn’t explain' when you mean that it is not necessary to explain.
To say that it was not necessary for something to be done on a particular occasion in the past, use didn't have to or didn't need to.
Fortunately, she didn't have to choose.
I didn't need to say anything at all.
➜ See need
strong belief
You use must to say that you strongly believe that something is true, because of particular facts or circumstances.
There must be some mistake.
Oh, you must be Gloria's husband.
Have to and have got to can also be used in this way, but not when the subject is you.
There has to be way out.
Money has got to be the reason.
You can use must with be and an -ing form to say that you believe something is happening.
He isn't in his office. He must be working at home.
You must be getting tired.
Be careful
Don't use must with an infinitive to say that you believe something is happening. Don't say, for example, ‘He isn’t in his office. He must work at home'.
To say that you believe something is not true, use cannot or can't. Don't use ‘must’ or ‘have to’ with not.
The two statements cannot both be correct.
You can't have forgotten me.
'must' also found in these entries:
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