Modal need is usually restricted to questions and negative sentences. Although people often say that lexical and modal need are interchangable, they have slightly different meanings. Modal need has a deontic flavour. This means that is used to talk about permission and obligation as opposed to pure necessity. Usually modal need implies that some particular person decrees or thinks that it is necessary to do something - or doesn't decree it necessary. This can have all sorts of subtle effects. Consider the following:
- You don't need to be there till after 5pm.
- You needn't be there till after 5pm.
The first is quite neutrally saying that it's not necessary for you to be there till after 5pm. The second is saying something like I don't find it necessary for you to be there till after 5pm or I don't oblige you to be there till after 5pm.
I would advise against using modal need in general, because it can have all kinds of subtle nuances that could cause you problems - unless you are very sure of what you're doing.
[Lexical words and function words:
Many grammars talk about function words, the kinds of words you learn about in grammar lessons, and lexical words, normal verbs adjectives nouns and so forth that you learn as vocabulary. Using these categories, modal verbs are considered function words, main verbs are considered lexical words].