want & need分別. 請用英文答

' want ' means looking forward to own something or achieve realization of something whereas' need' is related to a matter of necessity; for example,
you want a horse doesn't mean you need a horse. However, there are
situations where the two serve the purpose to define the same thing like
" I need some money.", " I badly want some money.".

They are interchangable most of the time, but I believe your question is asking the difference between them and more importantly under what situation, is one not replacable by the other.

Example : Past tense

If you say : He is a wanted criminal
it would be wrong to say : He is a needed criminal

Present tense
If you asked :
1) Do you want to play basketball?
2) Do you need to play basketball?

I would have answered :
ans1) Yes, I want to play basketball. (smile)

ans2) No, I don't need to play basketball. (disgusted)

The intonation of both questions are different, the first, seeked your company in playing while the 2nd, questioned your needs in playing.

want is a desire
need is a request

I want to do something - plan or wish to do something

He/Somebody wants to become the In-Charge of the shipping department .

----------------------------

I need to do something - must do something.

He /Somebody needs to work hard to becorme the IN-charge of the shipping department

--------------------------

He does not want to go to China but he needs to do because his manager tells him to do so.