The modal verb must is used with a bare infinitive to express the idea that something is necessary or obligatory or that something is certain or probable.
Need to and have to also express the idea of being necessary or obligatory.
The past tense of must is must, but it is used only in reported speech or in certain senses of the word. Had to is often used as the past tense of must. There are more details in the sections below.
Describing What Is Necessary
Must is used to say that something is required or necessary.
- These computer files must be given a name before they can be opened.
- In order to survive, an animal must have food and water.
- If you want to make a phone call from this phone, you must press that button.
- For the reaction to take place, the liquid in the test-tube must be heated to its boiling point.
Have to and, in slightly more informal English, need to can also be used in this sense:
- These computer files have to be given a name before they can be opened.
- The liquid in the test-tube has to be heated to its boiling point.
- These computer files need to be given a name before they can be opened.
- The liquid in the test-tube needs to be heated to its boiling point.
Have to can refer either to general facts or to single events:
- I have to catch the bus at 6.30 every morning.
- I have to catch the bus at 6.30 tomorrow morning.
- Do you really have to catch the bus at 6.30 every morning?
- I had to walk home last night since I had missed the last bus.
In informal British English, you can say have got to instead of have to. It can only refer to single events, and cannot be used to refer to the past:
- I’ve got to catch the bus at 6.30 tomorrow morning.
- Have you really got to catch the bus at 6.30 tomorrow? BUT
- Did you really have to catch the bus at 6.30 yesterday morning?
Must, have to and need to + not
There are important differences in meaning between must not, do not have to and do not need to. For example:
- You mustn’t press that button if you want to make a phone call from this phone
means that it is necessary for you not to press the button. Do not press the button, because if you do press it you will not be able to make the phone call. On the other hand:
- You don’t have to press that button if you want to make a phone call from this phone
means that it is not necessary for you to press the button in order to make a phone call. There is no reason to press the button because you can make a phone call without pressing it Do not need to and needn’t have this same meaning:
- You don’t need to press that button to make a phone call. OR
- You needn’t press that button to make a phone call.
Must can only refer to the present or the future. When referring to the past, you use had to or, more informally, needed to (remember that you cannot use the ‘have got to’ construction in the past tense either):
- The files had to be given a name before they could be opened.
- The liquid needed to be heated to its boiling point before the reaction could take place.
Must can refer to something that is necessary to get a result you want in the future:
- We must hurry if we are to catch the six o’clock train.
You can also use will have to (or, less commonly, shall have to – see page 178):
- We will have to hurry if we are to catch the six o’clock train.
If something is not necessary for a particular result, you can use do not need to:
- We don’t need to hurry. The train doesn’t leave for another hour.
Didn’t need to and needn’t have
When you say that you didn’t need to or didn’t have to do something, you mean that you knew that it was not necessary to do it (and it often implies that you didn’t do it):
- We didn’t need to hurry. We knew that rhe tram wasn’t leaving tor another hour.
- I didn’t have to buy milk There is an unopened bottle in the fridge.
On the other hand, if you say that you needn’t have done something, you mean that you did do it but then found that it had been unnecessary:
- We needn’t have hurried after all. The train was late.
- I needn’t have bought any more milk. We still have a litre in the fridge.
Must is used when saying that something cannot be avoided, that it is going to happen, or that there is no other possible option:
- We must all die some day.
- A line must be either straight or curved. It can’t be anything else.
In this sense, you can also use have to but not need to:
- We all have to die some day.
Must and have to may also express irritation at what someone has done:
- Why must you be so stupid?
- Of course, Susan has to go and tell her mother we were at a club last night, when I’d told my mother that we were staying in to study.
After why?, the question form is usually must…? Questions can also be formed with do … have to?:
- Do you have to be so stubborn?
The past tense is had to. The question form is did… have to?:
- Did you have to be so stubborn?
In a question, must and have to may be understood as making requests. For example:
- Must you go so soon?
- Do you really have to go?
These are really requests for the person being spoken to to stay.
Expressing what is Likely or Certain
Must is used to express what you think is likely or certain, often as a result of a process of thought or inference.
- There’s someone at the front door. That must be John now.
- You must be Shona’s mother. She looks Just like you.
- If these are your boots, then those ones must be mine.
- It must be nearly lunch-time.
- You know what American tourists say: if today’s Thursday, this must he London, and if it’s Friday, we must be in Paris.
The negative equivalent to must in this sense is can’t or cannot:
- That can’t be John yet. It’s far too early.
- This can’t be London. That’s the Eiffel Tower, so we must be in Paris.
When referring to the past, use must have:
- We’re lost. We must have taken the wrong road.
- The first humans must have crossed into Australia some 25,000 years ago.
When referring to the past with a negative, you use can’t have or couldn’t have:
- ‘It is surprising that John hasn’t come to my party.’
- ‘He can’t have got your invitation.’ (that is, he isn’t at the party, so I assume he has not received the invitation)
- ‘It was surprising that John didn’t come to my party.’
- ‘He couldn’t have got your invitation.’ (that is, he wasn’t at the party, so I assume he had not received the invitation)
In reported speech, use couldn’t have:
- We knew that he couldn’t have received the invitation.
Giving Commands or Instructions, and Expressing Intentions and Invitations
Must is used to tell someone what to do or what not to do, that is, in formally stating rules and regulations or in giving orders and expressing strong wishes.
- Children must be accompanied by an adult.
- Guests must vacate their rooms by 10 o’clock on the day of departure.
- You must stop hitting your little sister.
- The police must do something to stop street crime.
In less formal English, rules and regulations may be stated with have to:
- ‘What does that notice say?’
- ‘It says that children have to be accompanied by an adult.’
In the negative, use must not/have not to or do not have to/do not need to. Must not and have not to mean that it is essential not to do something while do not have to and do not need to mean that it is not essential to do something:
- Passengers must not talk to the driver while the bus is moving. (= do not talk to the driver)
- You don’t have to talk to the driver at all if you don’t want to. (= you can talk to the driver, but you are not obliged to)
- You mustn’t tell anyone what you’ve seen. (= do not tell anyone what you have seen)
- We’ve not to tell anyone what we’ve seen. (= we are not allowed to tell anyone)
- You don’t have to tell your mother where you are. (= you are not obliged to tell your mother where you are)
In the past tense, you use had to:
- Children had to be accompanied by an adult.
In the negative, say wasn’t to, not hadn’t to:
- I wasn’t to tell anyone what I had seen.
In reported speech, you can use must or had to:
- My doctor said I must (OR had to) lose some weight.
- His mother told him that he must (OR had to) stop hitting his little sister.
Must and needn’t generally imply that it is the person speaking who has the authority to give the commands, while have to and don’t have to imply that someone or something else is making the rules:
- You must wear a tie when you eat in a restaurant. (-1 am telling you to wear a tie)
- You have to wear a tie when you eat in that restaurant. (= that is the restaurant’s rule)
- You needn’t wear a tie when you eat in a restaurant. (= I am telling you that I don’t think it is necessary to wear a tie)
- You don’t have to wear a tie when you eat In that restaurant. (= the restaurant does not Insist that you wear a tie)
Must is used when you are describing something you know you ought to do and that you intend to do:
- I really must stop eating so many biscuits.
- I must pay all these bills today.
- We must book our holiday this weekend.
Must is used when you are describing something you think ought to happen and to make suggestions:
- The government’s priority must be to lower the crime rate.
- You must come round for dinner one evening.
- It’s a great book. One of his best. You really must read it.
- You mustn’t give up hope.
Must and have to are also used to express doubt or enthusiasm:
- John washing the car? That I must (OR have to) see!