Why do we sometimes use past-tense words when talking about the future?

For example: "Would you mind if i WENT home early?" or "Is it ok if we DIDN'T go to the movies tonight?". This point came up in my English class and its a difficult one to explain. I've been scouring the net but most sites just explain the grammar point and not WHY we use past tense in situations like these. Your help is so appreciated, thank you!