MUST MUSTN'T NEEDN'T

MUST MUSTN'T NEEDN'T 

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)

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.

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.
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.