I am right, aren't I? aren't I (not amn't I)
You have to go, don't you? you (do) have to go...
I have been answering, haven't I? use first auxiliary
Nothing came in the post, did it? treat statements with nothing, nobody etc like negative statements
Let's go, shall we? let's = let us
He'd better do it, hadn't he? he had better (no auxiliary)

 

Notice that we often use tag questions to ask for information or help, starting with a negative statement. This is quite a friendly/polite way of making a request. For example, instead of saying "Where is the police station?" (not very polite), or "Do you know where the police station is?" (slightly more polite), we could say: "You wouldn't know where the police station is, would you?" Here are some more examples:

  • You don't know of any good jobs, do you?
  • You couldn't help me with my homework, could you?
  • You haven't got $10 to lend me, have you?

 

Question tags with imperatives

Sometimes we use question tags with imperatives (invitations, orders), but the sentence remains an imperative and does not require a direct answer. We use won't for invitations. We use can, can't, will, would for orders.

imperative + question tag notes:
invitation Take a seat, won't you? polite
order Help me, can you? quite friendly
Help me, can't you? quite friendly (some irritation?)
Close the door, would you? quite polite
Do it now, will you? less polite
Don't forget, will you? with negative imperatives only will is possible

 off: http://englishcenter.blogfa.com/post-88.aspx