English has six tenses. Each tense has a PRECISE use. Here are the uses of the two past tenses:
Past tense expresses completed action in the past. It doesn't matter when you know it.
I saw that movie. I saw that movie three times. I saw it yesterday. I saw it last week. I saw it last year.
Past perfect tense is used to describe the EARLIER of TWO past completed actions.
By the time he arrived, I had already left. (arrive, leave; leave happened first)
By midnight, the carriage had turned back into a pumpkin. (midnight "arrives", turn; turn happened before midnight "arrived")