I don't know what you're on about.
我不懂你在谈什么。
Well, what do you know? Look who's here!
唷，真想不到！你看谁来啦！
She of all people should know the answer to that.
在所有的人中，唯有她最应知道那个问题的答案。
I don't know how you can say things like that.
真想不到你怎么会说出这种话来。
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。