A lot of opinions on the proper use of the past perfect seem quite contradictory. One that's been puzzling me is it's usage in the clauses that incorporate words such as "before" or "after", that clearly mark the chronology of events. Even though that could render the past perfect redundant, many workbooks still advise using it. Why?
I've always done this intuitively, and omitted more complex tense structures when they were not needed. As for now though, I'm not quite sure if I've been right the whole time.
I went there after I finished my classes. (had finished?)
He repaired many cars before he received his mechanic's license. (had repaired?)
I saw a documentary on the Whydah before we visited it in Providence. (had seen?)
We waited until they left. (had left?)
On the other hand, there are cases in which I'd opt for using the past perfect tense.
Francis Drake had worked for the British Navy before he became a pirate, so he exactly knew what to expect from them.
Is there any rule? Or are they mostly interchangable?