为什么【wèishénme】

为什么 asks for a reason. It goes after the subject and before the verb.

It can also open the sentence, which puts the whole question in focus rather than the subject.

Putting it at the end – nǐ你bù不qù去wèi shén me为什幺 – is wrong.

The answer takes 因为

Unlike English, Chinese often keeps 所以 in the full answer as well: 因为下雨，所以我没去.

为什么 against 怎么

Both can ask why. 为什么 wants a reason and carries no attitude. 怎么 adds surprise – the speaker expected something else.

A teacher asking 你为什么迟到 wants an explanation. 你怎么迟到了 sounds more like disbelief.

为什么不 as a suggestion