Ask for explanation on M5.0

[maqiuming.q]

Hi, in M5.0, verb and tense part,
Last Monday, Mary realized that she will have to spend all of that night rewriting her application because shedid not back up her files.

The correct answer writes"Last Monday, Mary realized that she would have to spend all of that night rewriting her application because shehad not back up her files."

My question is that whether"Last Monday, Mary realized that she had to spend all of that night rewriting her application because she did not back up her files" is ok? What is the difference between using simple past tense and past perfect tense here?

Thanks a lot for anyone who can give me some help.

[RonPurewal]

"did not" doesn't work in this context; that would have the same timeframe as the rest of the sentence. i.e., "did not" implies something that was true in the same timeframe when mary made the realization.

e.g.,
mary realized that she would fail the test because she was not prepared
--> this makes sense; mary was not prepared at the actual time when she realized she'd fail.

mary realized that she would fail the test because she had not prepared
--> this also makes sense. "had prepared" isn't the same as "was prepared"; "had prepared" describes the action of preparing for the test, while "was prepared" describes the resulting state of "readiness".
since we're now talking about an action that did (or, more accurately, didn't) happen prior to the timeframe of the sentence -- and that has a direct impact on the situation in that timeframe -- "had not prepared" makes sense.

mary realized that she would fail the test because she had not been prepared
--> doesn't make sense. if we're going to describe the state in which mary finds herself, that should happen in the same timeframe as the realization.

mary realized that she would fail the test because she did not prepare
--> also doesn't make sense, since the act of preparing must be prior to the timeframe of the sentence.
(this sentence only works if it has a different meaning -- i.e., that mary never prepared for tests. then, "she did not [ever] prepare" becomes a generality, rather than a statement of something that was true in one particular case.

--

"had to" would make sense if it was already "that night".
if mary made this realization during the day -- so that "that night" was still in the future -- then you'd need "would have to".
(this difference is too subtle for the gmat exam.)