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, ahead of the subject.

It never goes at the end of the sentence.

• × 你学中文为什么 → 你为什么学中文

In a negative question, 为什么没 asks why something didn't happen and 为什么不 asks why someone won't or doesn't do it: tā他wèi shén me为什幺méi没lái来 against tā他wèi shén me为什幺bù不lái来.

Answering with 因为

The answer starts with yīn wèi因为. In a full sentence, 因为 often pairs with suǒ yǐ所以 where English would use "because" alone.

为什么 or 怎么

Both can ask why. 为什么 wants a reason and carries no attitude. zěn me怎幺 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

为什么不 + verb is often a suggestion rather than a real question, like English "why don't you". The expected reply is agreement, not a reason.