All visitors must wear hard hats. All visitors must wear hard hats.
Must is a modal auxiliary verb which has different uses.
=====Construction=====
affirmative: subject + 'must' + bare infinitive You must go.
negative: subject + 'must' + 'not' + bare infinitive You mustn't go.
interrogative: 'must' + subject + bare infinitive Must you go?
Note that like other modal auxiliaries, must has no infinitive and we use the same form with 'I', 'you', 'he', 'she', 'it', 'we' and 'they'.
=====Uses=====
| Use | Examples |
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Necessity
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You must work hard if you want to succeed. The company must improve its sales figures next year.
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Obligation - when there is a rule or requirement.
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Visitors must report to reception. All drivers and passengers must wear seatbelts. All meetings must begin and end on time.
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Strong advice, including to oneself. This is stronger than should.
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You're ill and you're not getting better; you must go to the doctor's. I must remember to phone him back. |
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Deduction - when we believe something is certain.
The negative form is 'can't'.
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You must be tired - you work so hard all the time. That must be the postman at the door - I'm expecting a parcel.
"How old is she?" "Well, she started work a few years before me so she must be about 40."
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To talk about the best or right thing to do. [a strong opinion]
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Banks must tell customers in advance about charges. The government must spend less money. |
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To be emphatic with an invitation.
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You must come and see us sometime.
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Deduction
For deduction, the opposite of must is 'can't'.
That must be the postman at the door.
That mustn't be the postman at the door - it's too early. 
That can't be the postman at the door - it's too early. 
You can't be cold - it's 30˚ and you're wearing a coat.
It can't be easy having four young children and a full-time job.
'Must', 'should' or 'will' for deduction
These are used in the same way but with different levels of certainty. For example, someone hears a knock on the door at the time that the postman usually arrives and says:
That should be the postman. [It's probably the postman but it could be someone else.]
That must be the postman. [I'm certain it's the postman but there's a possibility that it isn't.]
That'll be the postman. [I have no doubt at all that it's the postman.]
=====Questions with 'must'=====
We are more likely to use 'have to' to ask a question because questions with 'must' have an additional meaning. They can suggest a rebuke, disapproval or disappointment:
Must you wear the same coloured shirt everyday? It's so boring.
Must we have another meeting? I've got much more important things to do today.
Must you go? We'll miss you.
In order to show similar strong feelings using 'have to', we can emphasise the word 'have' and maybe add 'really'.
Do I have to go with you, mum? I'd much rather stay here with my friends.
Do I really have to go with you, mum? I'd much rather stay here with my friends.
They have the same general meaning but are sometimes used in different ways.
For necessity, 'must' is often used in formal situations - spoken or written. For example, in a formal announcement or a written notice. In everyday conversation, we use 'have to'.
So, if you hear 'must' in conversation, it is being used emphatically or as advice, including to oneself.
For example:
'Must' for necessity [formal or emphatic] | 'Have to' for necessity [conversation] |
All visitors to this site must wear protective clothing. | "We have to wear protective clothing at work." |
ID cards must be shown before entering. | "Oh, look. We have to show our ID cards before we go in." |
"Sorry, but I must leave the meeting early today. Is that OK?" |
"Sorry, but I have to leave the meeting early today. Is that OK?" |
"Arnaud won't be at the meeting next week. He must go to Paris to see a client." |
"Arnaud won't be at the meeting next week. He has to go to Paris to see a client." |
"You really must eat less if you want to get into that dress for the wedding." |
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"You must finish that before Friday or we'll lose the contract."
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'Must' for advice - telling yourself what you should do. |
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"I must go to the bank today." [I should have gone last week but I've been busy. I'm going on holiday tomorrow so if I don't get my foreign currency today, it will be too late!]
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"I have to go to the bank today." [I have an appointment or I need to get some money, etc.]
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"I must stop drinking wine every evening - I'm not sleeping very well at the moment!"
| "The doctor told me I have to stop drinking wine while I'm taking these tablets." |
"I must start looking for a new job - I just don't enjoy working here anymore." | "I have to find a new job by January when this contract finishes."
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"I mustn't forget to call Louise today."
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Must can be replaced with 'have to' for the past or future. For example:
All delegates must arrive at the conference one hour before it begins.
Tom couldn't come to the meeting so we had to arrange another date.
We will have to postpone the launch until next month.
Expression | Meaning | Example |
You must be joking.
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When we don't want to believe what we hear or we want to refuse, or deny something strongly.
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"The phones aren't working again." "You must be joking! That's the third time this week.
"Do you like Juan?" "You must be joking; he isn't my type at all."
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Mustn't grumble
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A response to questions like "How are you?" that means everything is ok - not great, but not bad either.
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"Hi, how are things with you?" "Oh, not bad, mustn't grumble."
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Learners must complete their e-learning if they want to make progress.
You must pay a 10% deposit now and the balance in six weeks.
You mustn't eat so many burgers! You must eat more healthily.
I must try to relax and not appear too nervous.
My keys must be in here - I can't find them in the usual place.
The company must do something about these disappointing results. We must all make sure this doesn't happen again next year.
You mustn't park on double yellow lines in England.
This is great - you must come and see us again soon.