Past Perfect or Simple Past? Shit!

Wow! You can almost write a book about this subject. The best way to do this is to look at examples:

I've seen a lot of good movies recently, but the one I saw yesterday was not good.
Joe has just broken another window. Yesterday he broke the neighbour's window.
I've never been in Canada, but I was in the US in 1996.
The doctor hasn't arrived yet, but his secretary arrived before 8 o'clock.

As you can see, both refer to past events. We use the Perfect when the moment in which the event ocurred is not known or not relevant. However, if you state in the sentence the moment in which the event ocurred, you cannot use the Perfect. You then use the Simple Past. It is also interesting to note that the Simple Perfect in general corresponds to the Portuguese Passado plus the adverbs já, nunca, and ainda não:

Você já falou com ele? - Have you already talked to him?
Eu nunca vi uma coisa dessas. - I've never seen something like that.
Ainda não terminei o relatório. - I haven't finished the report yet.

If you use the Simple Past when you should have used the Perfect, you won't be totally wrong (especially in American English). It's just a matter of elegance. You can say: The doctor didn't arrive yet. It's OK, but it sounds more elegant if you say: The doctor hasn't arrived yet.