why

Why is the sky blue during the day and red at sunset?

(Chinese) 为什么天空在白天是蓝色的，而在日落时是红色的？

I don't understand why the flight was canceled without any warning.

(Chinese) 我不明白为什么航班在没有任何警告的情况下被取消了。

Before we fix the machine, we need to understand the why of its failure.

(Chinese) 在我们修理机器之前，需要弄清楚它故障的原因。

He explained the how and the why behind his difficult decision.

(Chinese) 他解释了他做出这个艰难决定背后的方式和原因。