I remember telling my friend that they would have been really busy preparing for the test had they been.
내 친구한테 걔네가 시험 준비하느라 무지 바빴을거라고 말한게 기억나, 대충 이거 같은데
뒤에 had they been은 왜 있는걸까요?

I remember telling my friend that they would have been really busy preparing for the test had they been.
내 친구한테 걔네가 시험 준비하느라 무지 바빴을거라고 말한게 기억나, 대충 이거 같은데
뒤에 had they been은 왜 있는걸까요?
