※ 引述《hopeliu (阿翔)》之铭言：
: [请益]国中文法问题
: Mary ____________ before Tom ____________.
: (A) had arrived left
: (B) arrive had left
: (C) arrived had left
: (D) had arrived leave
: 请教各位大大，这题答案A，
: C错的理由是？
这题ac都对 !
https://reurl.cc/70y795
We can use before with past simple OR past perfect to talk about an action
that happened before something else.
We arrived just before the plane doors closed.
Luckily, we arrived before the plane doors had closed.
https://reurl.cc/jWqQj2
Both of these sentences would sound equally good with a simple past in place
of the past perfect.
Furthermore, changing them in that way would have no effect on the sequence
of events. What it would do (what using the past perfect almost always does)
would be to set a temporal focus at some later point in the past.
https://reurl.cc/p3m904
Here are just a few quidelines:
Before I did X, I did Y - the tense for a simple record of the things one did.
Before I opened the door, I put the key in the lock.
Before I did X, I had done Y - the tense when one wishes to draw attention to
Y, and to lead the listener to an understanding of its relation to X.
Before I opened the door, I had looked carefully into the barn for snipers.
Before I did X, I had been doing Y - the tense when one wishes to draw
attention to Y, particularly when the fact that one had been doing Y throws
strong light on X - it also stresses the continuous element in Y, of course.
Before I took up codebreaking, I had been studying the mathematics of codes
for three years.
We mustn't suggest for a second that we can substitute the one for the other
without changing anything.