Unit 6- Must, mustn't, needn't

You MUST do something = it is necessary that you do it....

• example: Don't tell anybody what I said. You MUST keep it as a secret
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You MUSTN'T do something = it is necessary that you do not do it (so don't do it)

• example: It's essential that nobody hears us. We MUSTN'T make any noise 
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You NEEDN'T do something = it is not necessary that you do it, you don't need to do it

• example:We've got plenty of time. We NEEDN'T to hurry (it is not necessary to hurry)

Notes:

instead of NEEDN'T you can use DON'T /DOESN'T NEED TO

Remember that you can say : don't need to do but needn't do

Now compare this: needn't (do) and needn't have (done)

examples:

a) That shirt isn't dirty. You needn't wash it!

b) Why did you wash that shirt? it wasn't dirty. You needn't have washed it!