1: remain usually refers to (a lack of) changes in location, whereas maintain refers to (a lack of) changes in status. Remain: Please remain where you are. Please remain standing. This is all that remains after the explosion. Maintain: I would like to maintain my grades in school. I need to maintain my standing (meaning to continue having a good image). I still maintain that was a bad idea (I still believe that was a bad idea). As for reply and respond, they should pretty much always be interchangeable. Maybe in certain situations one term is more colloquial but I can't think of any time it would be wrong to replace one with the other.