John must do it for us, won't he? John must do it for us, right? even: John must do it for us, mustn't he?
He must have forgotten it, hasn't he?
Accidents will happen, won't they?
(the intonation, rising or falling, is important for what kind of answer you expect)
t.q. are constructed afresh for every sentence, and are therefore quite variable. Also, a tag question need not have the grammatical form of a question (will you?), since an adverb may serve the purpose instead: right? alright? surely? OK? But this would be informal.