Need(n't), dont have to and mustn't

Mustn't and needn't / don't have to
We use mustn't to say that something is NOT ALLOWED and needn't (or don't need to) or don't
have to to say that something is NOT NECESSARY:
• You mustn't walk on the grass here.
• You mustn't put anything on the shelves until the glue has set hard.
• They proved that watching a chess match needn't be boring.
• We needn't go into details now, but we seem to agree on the general principles.
Need, needn't, and don't need to / don't have to
Need can be used as a modal verb (before a bare infinitive) or as an ordinary verb. Compare:
• You needn't speak so loudly. (= modal verb)
• She needn't come with us if she doesn't want to. (= modal verb)
• She's thirsty. She needs a drink. (= ordinary verb)
• Jim and Bob are here. They say they need to see you urgently. (= ordinary verb)
When it is a modal verb need is most commonly used in negative sentences, although it is sometimes also used in questions:
• Need you go home so soon? (or, more commonly Do you have to go...?)
• Need I say more? (or, more commonly Do I have to say...?)
We can use either needn't or don't have to when we say that is unnecessary to do something:
• It would be good to see you, but g y , yyo u needn't ((or don't have to) come if you're busy.
• You needn't (or don't have to) whisper.

Didn't need to / didn't have to and need not have
When we say that it was not necessary to do something in the past, and it wasn't done, we use didn't need to or didn't have to. To show that we think something that was done was not, in fact, necessary we use need not have:
• Chris and June phoned to say that they couldn't come to eat, so I didn't need/have to cook dinner. (= I didn't cook the dinner)
• I needn't have cooked dinner. Just as it was ready, Chris and June phoned to say that they couldn't come to eat. (= I did cook the dinner) Study how we use need with scarcely, hardly, and only, particularly in formal contexts:
• We need hardly point out that there is a water shortage at the moment. (= it is almost unnecessary for us to point out...)
• I need scarcely add that you will be missed. (= it is almost unnecessary for me to add...)
• The changes need only be small to make the proposals acceptable.
'...hardly need to point out...,' '...scarcely need to add...' and '...only need to be...' are also possible, and less formal.