Sentence Correction

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.
or
if she had known about it.

i could not understand the following explanation to the problem
The original sentence contains the phrase would have gone, indicating that (1) Stacey did not go, but that 2) she surely would have done so if if she had known. The only way 1 and 2 can both be true is if Stacey did NOT know about the party. The phrase if she knew is therefore illogical here because it conveys the meaning that perhaps Stacey did know about the party. The bold face alternative uses the subjunctive mood (if she had known), appropriately indicating that Stacey did not know about the party.

i do not understand how the original sentence implies that Stacey knew about the party.
Can some one please explain.

There is some grammatical rules that GMAT testmakers love to test us.

If X happens, then Y will happen.
If X happened, then Y would happened.
If X had happened, then Y would have happened.

So the first sentence is wrong but the second sentence is correct.

I hope this helps.