by mschwrtz Thu May 13, 2010 4:53 pm
I'm moving this to another forum, since this one is devoted to MGMAT CAT questions, but it's a good question, so....
If you're using the 4th Edition, you'll find these examples on page 109, rather than page 46. I'm not sure what that first sentence would mean without the "had," because "The film started by the time..." isn't idiomatic. This sentence isn't meant as an example of the correct use of the idiom "by the time," but if you leave out the "had" you run into that issue.
The following are acceptable usages:
"She was gone by the time he woke up."
"She had left by the time he woke up."
The following are unacceptable usages:
"By the time he woke up she left."
"She left by the time he woke up."
There's a lot more to say about the expression "by the time," but until I hear of a real GMAT question that tests that idiom, I'm going to leave that other stuff unsaid.
About the sentence "The teacher thought that Jimmy had cheated on the exam": No, if you leave out the "had," it doesn't change the meaning in the way that you suggest. In fact, this is a case where you could leave out the "had" because the sequence is perfectly clear without it. Surely Jimmy's alleged cheating took place before the teacher's thinking. The past perfect is perfectly fine here, and the sentence is a good enough example of its use, but the simple past would also be acceptable.