First conditionnal :" If I slap him what will he do.?" We're questionning about a future where if the slap happens ( not happened yet) what the person in question will do.
Second conditional: " If I slapped him what would he do." We're questionning about a hypothetical future where the slap already happened and the consequences of the act Or if the slap were to happen (not happened yet but we're wondering if we do it what will happen meaning the same as first conditionnal ) ?