". The if-clause refers to something that happened before the then-clause, hence the past perfect.
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renan torres-riveroWhy, then, is the "would have" constructionThink of it as a mistake. Native speakers often make that mistake. Don't worry about it. Accept it as a substitute for had known when you hear it, but use had knownisused in the if-clause?
renan torres-riveroBut, that sentence was part of a GMAT test and the correct option is the one that is underlined. I chose the "had done" structure but was incorrect.Perhaps it would be helpful to write what the other options were.
renan torres-riveroI chose "D". It was wrong. They said "A" was the correct answer.