A. Must mustn’t needn’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 haven’t got much time. We must hurry.
‘You mustn’t do something’ = it is necessary that you do not do it (so don’t do it):
* You must keep it a secret. You mustn’t tell anybody else. (= don’t tell anybody else)
* It’s essential that nobody hears us. We mustn’t make any noise.
‘You needn’t do something’ = it is not necessary that you do it, you don’t need to do it:
* You can come with me if you like but you needn’t come if you don’t want to. (= it is not necessary for you to come)
* We’ve got plenty of time. We needn’t hurry. (= it is not necessary to hurry)
B. Instead of needn’t, you can use don’t/doesn’t need to. So you can say:
* We needn’t hurry. or We don’t need to hurry.
Remember that we say ‘don’t need to do’, but ‘needn’t do’ (without to).
Needn’t and don’t need to are similar to don’t have to (see Unit 31C):
* We’ve got plenty of time. We don’t have to hurry.
C. Needn’t have (done)
Study this example situation:
I think it’s going to rain. I’ll take the umbrella.
I needn’t have brought the umbrella.
George had to go out. He thought it was going to rain, so he decided to take the umbrella.
But it didn’t rain, so the umbrella was not necessary. So:
He needn’t have taken the umbrella.
‘He needn’t have taken the umbrella’ = He took the umbrella but this was not necessary. Of course, he didn’t know this when he went out.
Compare needn’t (do) and needn’t have (done):
* That shirt isn’t dirty. You needn’t wash it.
* Why did you wash that shirt? It wasn’t dirty. You needn’t have washed it.
D. Didn’t need to (do) and needn’t have (done)
I didn’t need to… = it was not necessary for me to… (and I knew this at the time):
* I didn’t need to get up early, so I didn’t.
* I didn’t need to get up early, but it was a lovely morning, so I did.
‘I needn’t have (done) something’ = I did something but now I know that it was not necessary:
* I got up very early because I had to get ready to go away. But in fact it didn’t take me long to get ready. So, I needn’t have got up so early. I could have stayed in bed longer.