Need needs to be a full verb, but needn't needn't

That's the rule.

Not good enough? O.K., here are some more details:

The heading already gave the game away: need is some sort of a hybrid. You can use it in two different ways. Even three, to be exact.

And what's the f...ine difference between must and need, then?

Not much, to be frank.

You could say, though, that you use need whenever you feel a necessity yourself. When others feel that you should do something, you would use must.
The negative form needn't or don't need to is a good way to express the negated form of must, because mustn't doesn't work (see must in the modals table).

Boy and girl

I need to clean up before she comes to my place tonight.
You can see yourself it is necessary.

Husband and wife

I must clean up my desk even though I still can find things (sometimes).
You just surrender to prevent petty squabbles. You simply know your desk is perfectly OK.

You need to click here to go back to the modals table.