-면 is a conditional ending, and -다 is related to reported speech. I d say there are two basic uses of 다면. One is a sort of quotation of what someone says or said, the other is a contrary-to-fact conditional. E.g.: 비가 온다면 ... If it rains (as you/she/they say(s)), (then) ... 내가 만약 의사가 된다면 ... If I were to become a physician... Thanks if someone can check... ^^