5Please help me with this question: when I ask somebody 'did you use to be a teacher?', I must have known that he or she isn't a teacher now, otherwise it is inappropriate to ask this question?
Yes, unless you are being sarcastic about a teacher who is, in your opinion, past their best.
Is it true for all similar questions? 'Did you use to live in Taiwan?' implying that I know you don't live in Taiwan at the moment of speaking? Yes