I don't understand what you are talking about.

我不明白你在说什么。

《柯林斯英汉双解大词典》

I hear what you are talking about.

我听到你们在说什么了。

youdao

OK, I understand what you are talking about.

好的，我明白你在说什么。

youdao

"I was in a long distance relationship for about a year, and Skype really helped because you could actually see the other person when you are talking."

VOA: special.2009.09.14

And you will find this pattern useful for when you are talking about something in your memory.

当你在谈论记忆中的某事时，这个句型非常有用。

remind me of 课堂 - SpeakingMax英语口语达人

It depends on what you are talking about.

这取决于你在说什么。

斯坦福公开课 - 经济学课程节选

Oh, you are talking about stadiums that need to be built in the future?

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To me it seems like you are talking about something you know extremely little about.

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There you are talking about fines rather than taxes, but the distinction is often pretty thin.

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