If you need something, or need to do something, you cannot successfully achieve what you want or live properly without it.
If an object or place needs something doing to it, that action should be done to improve the object or place. If a task needs doing, it should be done to improve a particular situation.
If there is a need for something, that thing would improve a situation or something cannot happen without it.
If you say that someone needn't do something, you are telling them not to do it, or advising or suggesting that they should not do it.
If you tell someone that they needn't do something, or that something needn't happen, you are telling them that that thing is not necessary, in order to make them feel better.
You use needn't when you are giving someone permission not to do something.
If something need not be true, it is not necessarily true or not always true.
If someone needn't have done something, it was not necessary or useful for them to do it, although they did it.
You use need in expressions such as I need hardly say and I needn't add to emphasize that the person you are talking to already knows what you are going to say.
You can use need in expressions such as 'Need I say more' and 'Need I go on' when you want to avoid stating an obvious consequence of something you have just said.
People in need do not have enough of essential things such as money, food, or good health.
If you are in need of something, you need it or ought to have it.
If you say that you will do something, especially an extreme action, if need be, you mean that you will do if it is necessary. In British English, you can also say if needs be .
You can tell someone that there's no need for them to do something as a way of telling them not to do it or of telling them to stop doing it, for example because it is unnecessary.
You can say 'Who needs something?' as a way of emphasizing that you think that this thing is unnecessary or not useful.