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》

I don't know why I settled on Miami, but anyway I did.

我不知道为什么我落户到了迈阿密，但不管怎样我做了。

《柯林斯英汉双解大词典》

Everybody answered the questions well, intelligently, while he - why, dear me - he did not know anything, so to speak.

VOA: special.2009.01.03

Now why--and here is the question why do we get this nervousness about the relationship between what I know and how I know it arising at this moment?

问题是,为什么那时候我们对,我知道什么和我如何知道的关系如此神经紧张？

耶鲁公开课 - 文学理论导论课程节选

So, hedge funds do not advertise, that's why people don't know about them and they have a mysterious -they sound mysterious.

对冲基金公司做不了广告,那也是公众并不了解他们的原因,他们披着一层神秘的面纱...,他们听起来挺神秘的

耶鲁公开课 - 金融市场课程节选

And I was - I often would wonder why - you know, why we don't correct these things, why didn't he correct these things.

NPR: Storied Glee Club Faces Waning Membership

But if you have the technical know-how, why not try your hand at creating the next great gadget or electronic service?

FORBES: Out-of-Work Execs: Start a Company, Create Jobs, Lead a Comeback

Ms. MESA: I really would to know why - where did they get the problem or where does it come from.

NPR: Calif. Program Helps Rid Triggers of Asthma