麻煩你了

[meatie]

Do I say "麻煩你了" before someone does me a favor or after someone did me a favor?

[abcdefg]

Deleted -- I'm not a native speaker. How I do it matters little and could be misleading.

[shuoshuo]

Before. E.g. 麻烦你帮我。。。

[Divato]

It's uncertain. You use 了 so there are two circumstances.
1. You asked for a favor and he said yes but he didn't do it yet.
2. He did you a favor.
It's supposed he have or had to try and pay effort to do you a favor. The moment he promises you a favor, you should consider getting him into your trouble so you may say 麻烦你了 after both circumstances.

If you say 麻烦你帮我....., it means you are asking for favor.

[aone]

You can say it both before and after,depending on the circumstance. e.g.

 

You borrowed a book from FANG. FANG needs it back tomorrow, but you are fully occupied, then you turn to YUAN for help and ask her to take the book to FANG. If YUAN agrees to do so, you can say -  那就  麻烦你了。In this case, YUAN will do you a favor to take the book to FANG, so before YUAN does it, you say 麻烦你了。

 

The next day, YUAN gives the book to FANG, then, FANG can say 麻烦你了.