Lately, the difference between both tenses is disappearing, mostly in South America, but there is actually a different meaning:
- Mi madre me dijo que nunca envidie a nadie. -> This is a principle that is valid for me today and for the future as well, in case I meet someone that would have something I would like to have.
- Mi madre me dijo que nunca envidiara a nadie. -> She told me this in the past, even if I still live by it. I might just think I will never be jealous of anyone, no matter what they have.
Some more examples:
- Te pedí que no le digas a nadie. -> I asked you before not to tell anyone, and you still might tell, so I'm asking you again not to tell.
- Te pedí que no le dijeras a nadie. -> You already told someone else what I asked you to keep to yourself.
- Abrí la puerta para que entren. -> I opened the door for someone who would come in as I opened it, but someone may still come in.
- Abrí la puerta para que entraran. -> Someone was supposed to come in in the past and I opened the door for them. I don't expect anyone else to arrive.
- Te castigué para que aprendas. -> I expect you will learn something from being grounded.
- Te castigué para que aprendieras. -> I expected you had learned something from having been grounded, but I see you didn't.
Some cases when it's plain wrong to mix past and present:
- Esperábamos que
des dieras tu opinión.
- No sabía que
conozcas conocieras la ciudad.
-> In these cases, the imperfect calls for the past subjunctive. It won't admit the present.
- Se fue sin que le
digas dijeras nada. -> Here, there is no possibility that you can say anything to someone who is already gone, so no present.
