NEED(N´T), DON´T 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 watchig 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...?)
* 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 you needn´t (or don´t have to) come if you´re busy.
* You needn´t (or don´t have to) whisper. Nobody can hear us.

To give permission not to do something we can use either needn´t or don´t need to:
* You needn´t cut the grass, I´ll do it later.
* You don´t need to cut the grass, I´ll do it later.

To talk about a general necessity, we prefer don´t need to:
* You don´t need to be over 18 to get into a disco.

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 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 dinner)

Source: Advanced Grammar in Use - Martin Hewings - Cambridge.