it's origin? I guess it's a matter of linguistics. In persian (my native language) we also use past tense for such conditional sentences but we don't know why! This is a very interesting question that you raise but do your students really need your explainations?
A: You can't go into the room.
B: Why not? If it is my turn, I (will) go.
A: I am sorry that you should wait.
B: Yes. If it was my turn, I would go, but it isn't my turn.
perhaps you just need 3-4 of these examples to make them understand the difference.
Once upon a time there was man with a long beard. One day somebody asked him:"when you are sleeping do you cover your beard with the blanket or you spread it on the blanket?". Since that time he couldn't sleep well.