1, Couldn't you do a favour and leave me alone?
2, Could you do a favour for me?
3, I want you to do a favour for me.
4, Who is Mary go to do a favour for?
5, I hope i can do a favour.
6, It's quite common, if someone agrees to do a favour, to use the future tense to promise a reward. So for example, you might say.
7, But he browbeats however: I will answer birthplace tomorrow, see you still can do a favour for who!