The word "need" appears as a modal verb infrequently, but it does happen. If used as a modal, it would be associated with a main verb, much the way the word "must" is used. For example, "you need not go".

The simple answer is, look at how other modal verbs are used and look at how regular verbs are used. You'll see the difference immediately. The very simple answer is, unless you're a very advanced user of English, you needn't worry about it.

I believe "need" isn't used as a modal verb anymore... Well, at least in the USA. hmm