Another question:

Can you please tell me why this sentence is correct?

The brothers had completed their chores before their mother came home.

I think that since we are using "before" we shouldn't use Past perfect "had completed". right?

Even i feel same. What's the source?

voodoo_child wrote:Another question:

Can you please tell me why this sentence is correct?

The brothers had completed their chores before their mother came home.

I think that since we are using "before" we shouldn't use Past perfect "had completed". right?

Yeah, an abbreviated summary of the rule on that is basically that you can do either -- past perfect or simple past. Some "rulemakers" argue that you do need the past perfect there because you don't have SPECIFIC time markers (like "at 8:00 pm"). Some other "rulemakers" argue that you don't. I think I personally would just use simple past in that sentence, but again, there's no consensus that makes one choice right and the other wrong. I'd be pretty shocked if the GMAT gave you a sentence that required you to make a call on past perfect in a "before" sentence, because there'd be an uproar -- that's the kind of question that wouldn't make its way onto the test because it wouldn't be bound to one correct answer (unless all the other answers were blatantly wrong for other reasons). And in writing in "real life," I'd use whichever you prefer in this construction.