你为什么学汉语？

Nǐ wèishénme xué Hànyǔ?

Why are you studying Mandarin?

你为什么这么生气？

Nǐ wèishénme zhème shēngqì?

Why are you this angry?

我不知道为什么今天餐馆不开门。

Wǒ bù zhīdào wèishénme jīntiān cānguǎn bù kāimén.

I don't know why the restaurant isn't open today.