Must / mustn't /needn't

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Must / mustn't /needn't

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.
  • We've got plenty of time. We don't have to hurry.

C. Needn't have (done)

Study this example situation:
George had to go out. He thought it was going to rain, so he decided to take the umbrella.
George: I think it's going to rain. I'll take the umbrella.                                 
But it didn't rain, so the umbrella was not necessary. So: He needn't have taken the umbrella.
George: I needn't have brought 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.


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