I'd prefer you left it open.
Great observation. I never knew that about my native language. It's stylistic, I believe. I am sure someone has said, "I'd prefer you leave it open." but the use of the past tense for other people's actions is the norm. "I would rather you stayed here." "I would prefer you kept that to yourself." but "I would prefer to stay here." and "I would prefer to keep it to myself."
Maybe it has something to do with describing ourselves having a preference when we perform an action and describing ourselves reacting after someone else's act.
Actually, I think there is also the phrase: "I would prefer it if I didn't have to wait." which is slightly different from "I would rather not wait.¨ and "I would prefer not to wait." but essentially the same.
I would just explain these verb preferences as stylistic. Not following this past tense "rule" won't make you hard to understand. Someone may just think it sounded a little unusual.
In this case the past tense "left" is used as a kind of subjunctive after the verb "prefer". In other languages like French or Spanish one would mandatorily use a subjunctive here ("prefiero que no vengas" = "I would prefer that you did not come").
Not sure about that, but maybe old or middle English had subjunctive forms that got progressively lost, and what remains is this hidden subjunctive with the form of a past tense? Can anybody knowledgeable confirm?