NEED, NEEDN´T, DON´T NEED TO...

NEEDN´T (NEED NOT)

I needn´t (do something) = it is not necessary to do it, I don´t need to do it:
* I needn´t clean the windows. They aren´t very dirty.
* You needn´t go to the bank today. I can give you some money.

You can also say don´t need to... (= needn´t)
* I don´t need to clean the windows.
* You don´t need to go to the bank today.

Source: Essential Grammar in Use - Raymond Murphy - Cambridge




NEEDN´T
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.


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.

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.

Source: English Grammar in Use - Raymond Murphy - Cambridge