I know what you're talking about. 我知道你在說什么。
This is exactly what I'm talking about. 這就是我想說的。
You know who I'm talking about. 你知道我說的是誰。
I have no idea what you're talking about. 我搞不清你說的東西。
I saw you out with another woman. 我看見你和一個女人出去了。
I don't know what you're talking about. 我不知道你在說什么。