Drained. And you don't know why.

而你不知道这是为什么。

youdao

And you don't know why.

而你却不知道是什么原因。

youdao

You don't know why they weren't interested in you.

你不知道为什么他们对你没有兴趣。

youdao

What, you think I regret saving you? I can see that you do.I just... I don't know why.

VOA: standard.other

If you don't know why you're about to eat cake like that you will soon, but you know this.

如果你现在还不知道为什么将要吃到那样的蛋糕,很快就会知道的，你清楚这一点。

哈佛公开课 - 计算机科学课程节选

but your ankle is just bothering you for some reason you don't know why

你的脚踝一直影响你的发挥，但你不知道为什么。

That's why 课堂 - SpeakingMax英语口语达人

"You don't know why that happened, " he told reporters at the London event.

BBC: Justin Bieber 'early' for second London gig

You don't know why they are here but they are here.

WSJ: Jason Gay: Water Cooler Topics Are Overflowing

You say you don't know why we are fighting -- what we are trying to accomplish -- and even whether we are "at war" at all.

CNN: What Obama should say about Libya