TESOL/Using need

The word "need" has a literal, informational meaning, and besides that it also accomplishes several other communicative tasks.

Mechanics
Usually "need" is used as a normal verb to express necessity.

Modal verb
Need can be a modal verb. It is more common in British English.
 * You needn't come to that meeting.
 * Need I say why?
 * That is the only meeting you need attend. You need attend that meeting.

Passive infinitive
"Need" plus a verb with -ing has the same meaning as a passive infinitive.

Regret
Saying someone need not have done something suggests that there was no reason to do it, that it was a bad idea. It differs in that respect from "did not need to have done."

Including the word "never" adds emphasis: someone need never have done something.

Commands
Using "need" in the present tense is often used to talk about obligation in the future. In particular, it can be used to give commands indirectly.
 * You'll need to finish your homework.
 * You'll need to come to my office.

Permission
Using "need not (do something)" grants permission not to do something at that moment. In this case, "need" is a modal verb. In a case where the listener might assume that they are obliged to do something, "need not" explicitly removes that obligation. This differs from using "need" as a regular verb, as in "You do not need to do something."

Possibility
"Need not" may also be used to equal "might not be." That is, it means that the possibility is less than 100%.


 * I'm sure she went to the wrong place. - She need not have: maybe she is just running late.
 * That face must mean that he hates me. - He needn't feel that way about you. Maybe he just remembered something unpleasant.