Good question - a few thoughts here:
-The most common way that past-perfect is used on the GMAT is when it is uncalled for and should be eliminated. If you feel fully comfortable knowing when you should not use past-perfect, you're in pretty good shape for verb-tense-related SC questions.
-From my experience not just with the GMAT but in speaking English for 30-some years, it's pretty rare that the past-perfect tense is required. Even when there is definitely a sequence of past events, simple-perfect works in most cases:
Example: Barack Obama was a community organizer in Chicago before he launched his political career and ultimately became president.
That's completely fine, and honestly it's a little awkward to say "Obama had been a community organizer in Chicago before he launched his political career." Where past-perfect becomes a little bit more necessary is if the purpose of the sentence is to distance one event further in the past to specifically designate the time shift:
Barack Obama had been content to work as a community organizer, but his success prompted many to encourage him to run for higher office.
Here, we're trying to denote that his previous career (and his happiness with it) took place long ago and that intervening factors changed moving into the nearer-term past.
-If the earlier action was not completed before the second action, past-perfect doesn't work:
For example, you would say "My grandfather was a carpenter until the day he died." and not "My grandfather had been a carpenter until the day he died." He was a carpenter at the same time that he died (sorry to be morbid...), so because that action wasn't completed we wouldn't use past-perfect.
This comes into play in your example: the brown sparrow was being slaughtered up until it became extinct, so the past-perfect doesn't really work there because one action led directly into the other. We could say:
"The passenger pigeon had once been the most abundant bird in North America for centuries, but in 1919 it became extinct after decades of indiscriminate slaughter."
In that sentence, our goal is to show that the previous condition (it had been numerous) took place well before - and had ended before - the second condition.
In summary - past-perfect is a little bit more than just "the first of two past events". It's used when there's a pressing need to set one event back further in the past than the other, and when that action has completed. Maybe just as importantly, the GMAT is much more apt to ask you about a case in which the past-perfect is pretty clearly uncalled-for, so your current understanding of the distinction should still serve you well.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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