I know you did it—I just want to know why.

我知道这是你干的，我只是想知道为什么。

《牛津词典》

I don't know why.

我不知道为什么。

《柯林斯英汉双解大词典》

"I don't know why they left," he said dolefully.

“我不知道他们为什么离开，”他悲伤地说。

《柯林斯英汉双解大词典》

If you visit the western mountain and plains state of Montana you will know why it is known as Big Sky Country.

VOA: special.2009.08.16

And I don't know why, but since time immemorial that is the way computer scientists have drawn trees.

叶子是在最下面的，我不知道为什么,但从记事以来他们就是这么画树的,这也许就是为什么我们。

麻省理工公开课 - 计算机科学及编程导论课程节选

But for me, I like to look at something and just say, "I like this. I don't really know why though."

但对我来说，我喜欢看着某个东西，然后说，“我喜欢这个。但我也不知道为什么。”

关于现代美术 - SpeakingMax英语口语达人

Galligan said recently that he didn't know why his former client wanted to represent himself.

NPR: Fort Hood Suspect To Represent Himself At Trial

We don't know why, but there is something stylishly simple, yet futuristic about it.

ENGADGET: Alt-week 7.21.12: Outer space, flying hotels and federal trolls

Many parents still want to know why so many fell down while other buildings remained standing.

BBC: China marks Sichuan earthquake anniversary