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。
I don't know how she puts up with him.
我不明白她怎么受得了他。
Does he know to come here first?
他知道要先到这儿来吗？
Had I known that I would never have come.
要是早知道，我绝不会来的。