I was recently asked which of the following was correct:
(a) Before I left the room, I switched off the lights. (simple past)
(b) Before I left the room, I had switched off the lights. (past perfect)
My reply was that (a) was correct, but I was told that (b) was given as the recommended answer in a primary school English exercise. However, I have no doubt that (b) is definitely wrong.
In Standard English (StdE), the simple past is used for events that may be represented as a single point on a time line showing present time and past time. It tends to be used when there is a specific time expression, e.g. yesterday, three hours ago. If we represent this on a time line, we need two points: one indicating present time, and another indicating yesterday or three hours ago.
The past perfect, by contrast, is used when referring to the earlier of two or more events. If on a time line we have three points A, B and C, where C is present time, the later past event B will be expressed in the simple past, whereas the event that preceded it, Event A, will be expressed in the past perfect.
The past perfect is properly used below:
(c) He popped by, but I had already left the room.
In Singapore, the past perfect is often incorrectly used, perhaps because of a poor understanding of what constitutes an ‘earlier past event’. Admittedly, it is not always easy to determine what an earlier past event is, and the proposition expressed in (a) and (b) is a good case in point: doesn’t switched off the lights qualify as an earlier past event, since it took place before left the room?
The explanation is not so straightforward, and lies in the fact that before I left the room counts as a specific time expression — hence the use of simple past switched as in (a) rather than past perfect had switched as in (b). Note that before I left the room is a subordinate clause; it cannot stand alone and has to be attached to a main clause, in this case I switched off the lights. Note also that, as is typical of subordinate clauses, it can be moved around: I switched off the lights before I left the room means the same thing. Another thing: it is an adverbial, like so many subordinate clauses are, and as it conveys time, it is a adverbial of time.
Contrast this with (c), which has two main clauses, He popped by and I had already left the room, either of which can stand on its own. Unlike subordinate clauses, main clauses cannot be inverted freely, hence *I had already left the room but/and he popped by is ungrammatical, or at best very odd. Hence, either main clause expresses an independent past event; neither serves as a time adverbial to the other.