I know what you're talking about. 我知道你在说什么。

　　This is exactly what I'm talking about. 这就是我想说的。

　　You know who I'm talking about. 你知道我说的是谁。

　　I have no idea what you're talking about. 我搞不清你说的东西。

　　I saw you out with another woman.  我看见你和一个女人出去了。

　　I don't know what you're talking about.  我不知道你在说什么。