must
[noun]
Synonyms: necessity, essential, fundamental, imperative, prerequisite, requirement, requisite, sine qua non

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Related Idioms: have got to, must needs
Related Words: ought, should want

[TahlilGaran] English Synonym Dictionary ▲

I. must¹ S1 W1 /məst; strong mʌst/ modal verb (negative short form mustn’t)
[Language: Old English; Origin: moste, from motan 'to be allowed to, have to']

1. (past tense had to) to have to do something because it is necessary or important, or because of a law or order ⇒ have to, oblige:
All passengers must wear seat belts.
It’s getting late. I really must go.
You must work hard.
We must all be patient.
Must I pay now?
For the engine to work, the green lever must be in the ‘up’ position.
Accidents must be reported to the safety officer.

REGISTER
In everyday English, people usually say someone has to or has got to do something rather than say they must do something, which can sound slightly formal or emphatic:
Everyone has to wear a seat belt.
I‘ve got to go at four o’clock.
Do I have to pay now?

2. used in negative sentences to say that something should not happen, because of a rule or law or because of the situation:
You mustn’t talk to your mother like that.
This book must not be removed from the library.
We must never forget how much we owe to these brave men.
No one must disturb him while he’s sleeping.
GRAMMAR
Do not use must not to say that it is not necessary for someone to do something. Instead, use need not or do not have to:
You need not (NOT must not) work through the exercises in order.

3. used to say you think something is very likely to be true or very likely to have happened:
Sam must be nearly 90 years old now.
His new car must have cost around £20,000.
You must have been really upset.
There must be something wrong with the engine.
Karl must’ve seen ‘Star Wars’ six or seven times.

4. especially British English spoken used to suggest that someone should do something, especially because you think they will enjoy it or you think it is a good idea:
You must come and stay with us in London sometime.
‘We must do this again,’ he said. ‘I’ve enjoyed it thoroughly.’

5. especially British English spoken used to say that you intend or want to do something:
I must call her tonight.

6. I must admit/say/confess spoken used to emphasize what you are saying:
I must say, it gave me quite a shock.

7. (why) must you ...? spoken used to tell someone that their behaviour upsets or annoys you:
Must you spoil everything?
Why must you always be so suspicious?

8. a must-have/must-see/must-read etc informal something that is so good, exciting, or interesting that you think people should have it, see it etc:
The exhibit is a must-see for anyone interested in Japanese art.
a must-read novel

9. if you must (do something) spoken used to tell someone that they are allowed to do something, but that you do not approve of it or agree with it:
All right, come along, if you must.
If you must smoke, please go outside.

10. if you must know spoken used when you answer a question that you think someone should not have asked, because it is slightly impolite:
Well, if you must know, I’m thirty-six.
⇒ you must be joking at joke²(2)

[TahlilGaran] Dictionary of Contemporary English ▲

II. must² /mʌst/ noun [countable usually singular]
[Date: 1800-1900; Origin: ⇒ must¹]
something that you must do or must have:
Warm clothes are a must in the mountains.

[TahlilGaran] Dictionary of Contemporary English ▲

must
modal verb
1.
BAD: People are not as careful as they must be and drop their litter in the streets.
GOOD: People are not as careful as they should be and drop their litter in the streets.

Usage Note:
Use must (or have to ) when, for example, there is a law or rule and you are not free to choose or decide for yourself: 'Candidates must answer all the questions in Part A and two questions in Part B.'
Use should (or ought to ) when, for example, someone advises you to do something but you are free to choose or decide for yourself: 'At the end of the examination, you should check your answers.'

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2.
BAD: The pupils mustn't go to the meeting if they don't want to.
GOOD: The pupils needn't go to the meeting it they don't want to.

Usage Note:
Use must not/never when you mean that it is essential that someone does not do something: 'The door to the X-ray room must never be opened when the red light is on.'
When you mean that it is not necessary for someone to do something, use needn't or don't need/have to : 'You needn't pay now. You can wait until the furniture has been delivered.'

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3.
BAD: You must be pleased to hear that I've already got the tickets.
GOOD: You'll be pleased to hear that I've already got the tickets.

Usage Note:
When you inform someone of something, use will be + pleased/interested + to hear/know/learn (NOT must ): 'You will be pleased to know that your old friend Peter has been promoted to Associate Professor.'

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4.
BAD: If you can't find her, she must hide somewhere.
GOOD: If you can't find her, she must be hiding somewhere.

Usage Note:
When you do not actually know where someone is or what they are doing, but certain facts allow you to guess, use must be or must be doing : 'If she isn't in her office, she must be in the canteen.' 'If she's in the canteen, she must be having her lunch.'

[TahlilGaran] Dictionary of Common Errors ▲