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.

Need + verb -ing	Passive infinitive
The dishes need washing.	The dishes need to be washed.
My sheets need changing.	My sheets need to be changed.
The bushes need trimming.	The bushes need to be trimmed.

Usage

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."

Example	Meaning
You needn't have given me that!	(I don't need your gift or I don't want it.)
You didn't need to give me that!	(I am flattered to receive your gift.)

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

Example	Meaning
You needn't have told me.	(I didn't want to hear that.)
You need never have told me.	(Hearing that was very upsetting.)

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."

Example	Meaning
You needn't pay for coffee.	Put away your wallet.
You do not need to pay for coffee.	It's free.

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.