A. must and mustn’t
You must do something = it is necessary that you do it:
- Don’t tell anybody what I said. You must keep it a secret.
- We don’t have much time. We must hurry.
You mustn’t do something = don’t do it:
- You must keep it a secret. You mustn’t tell anyone. (= don’t tell anyone)
- We must be very quiet. We mustn’t make any noise.
B. needn’t and don’t need to
You needn’t do something = it’s not necessary to do it (but you can if you want):
- We have plenty of time. We needn’t hurry. (= it is not necessary to hurry)
- Joe can stay here. He needn’t come with us. (= it is not necessary for him to come)
You can also use don’t/doesn’t need to:
- We don’t need to hurry.
Note that we say ‘don’t need to do’, but ‘needn’t do’ (without to).
Compare needn’t and mustn’t:
- You needn’t tell Steve. I can tell him myself. (= it is not necessary)
- You mustn’t tell Steve. I don’t want him to know. (= don’t tell him)
C. needn’t have (done)
Study this example situation:
Paul and Sarah reserved a table at a restaurant.
But when they went to the restaurant, it was almost empty.
They needn’t have reserved a table.
This means they reserved a table, but now they know this was not necessary.
Compare needn’t (do) and needn't have (done):
- Everything will be OK. You needn’t worry. (it is not necessary)
- Everything was OK. You needn’t have worried. (you worried, but it was not necessary)
D. needn’t have (done) and didn’t need to (do)
He needn’t have done something = he did it, but now we know that it was not necessary:
- Why did he get up at 5 o’clock? He needn’t have got up so early. He could have stayed in bed longer.
He didn’t need to do something = it was not necessary to do it. It doesn’t matter whether he did it or not:
- He didn’t need to get up early, so he didn’t.
- He didn’t need to get up early, but it was a beautiful morning, so he did.
You can also say, ‘He didn’t have to get up’ in these examples.
Try this exercise to test your grammar.
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B1-B2 Grammar : Must, Mustn't, Needn't
Choose the correct word.