According to the instructions we need to choose between either of the two alternatives as correct or propose another response to correct the sentence.

Stacey would have gone to the party if she knew about it.
if she had known about it.

i could not understand the explanation to the problem that suggests that the sentence as it stands implies that Stacey knew about the party. I feel that the sentences is correct as it stands.

if possible please explain further as i can't catch the author's train of thought.

This is a tricky issue. Most English speakers would probably be able to select the right form without understanding the grammar behind it.

Suppose Robert left my house an hour ago to go to the market. I could say:

If Robert took street A, it would take him 5 minutes to get to the store.
If Robert took street B, it would take him 10 minutes to get to the store.
(If PAST, then CONDITIONAL)
Here, we're discussing the options of possibilities for Robert to get to the store. these are not hypothetical situations, but possible scenarios.

However, when we're talking about hypothetical situations that never happened, we would use a subjective construction.

If Robert had taken street A instead of street B, then he would have gotten there five minutes earlier.
(If PAST PERFECT, then CONDITIONAL PERFECT)

Here, clearly, Robert did NOT take street A, so the entire scenario is hypothetical. You can find this construction on page 113 in the 4th Edition of the Sentence Correction Strategy Guide. (Case 5)

So in your sentence:
Stacey would have gone to the party if she had known about it.
(CONDITIONAL PERFECT, If PAST PERFECT)
is correct since this is a hypothetical situation that never happened.

_________________
Ben Ku
Instructor
ManhattanGMAT

i can't seem to find (even outside of the mgmat SC guide) the conditional for you used here:

If Robert took street A, it would take him 5 minutes to get to the store.
If Robert took street B, it would take him 10 minutes to get to the store.

if simple past, would + infinitive

is this a valid conditional construction? what about just using

if present, then present?

both of these valid. this is explained in our strategy guide on pages 112 and 113..

_________________
Tim Sanders
Manhattan GMAT Instructor

thanks i got simple past confused with "hypothetical subjunctive"

and you are right, it's right in the guide.

-Dave

Great, thanks Tim! :-)

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Jamie Nelson
ManhattanGMAT Instructor