Page 76 - English Grammar in Use

Unit
32 must mustn’t needn’t

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:

Can I reserve a table for two?
Paul and Sarah reserved a table at
a restaurant.
We needn’t have reserved a table.
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.

must (‘You must be tired’) ➜ Unit 28 have to and must ➜ Unit 31
64 Modal verbs (can/could/will/would etc.) ➜ Appendix 4 American English ➜ Appendix 7