You use then at the end of a topic or at the end of a conversation. `I'll talk to you on Friday anyway.' --- `Yep. Okay then.'
in that case or as a consequence; "if he didn't take it, then who did?"; "keep it then if you want to"; "the case, then, is closed"; "you've made up your mind then?"; "then you'll be rich
You use then in conversation to indicate that what you are about to say follows logically in some way from what has just been said or implied. `I wasn't a very good scholar at school.' --- `What did you like doing best then?'
now and then: see now there and then: see there. used when mentioning the person who had a particular job, title, or position at a time in the past