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。
She didn't know what life in a foreign country would be like.
她不知道外国的生活会是什么样。