Only when I put myself in their position they were more miserable than me.
A. I had known
B. had I known
C. I knew
D. did I know
网友回答
A. I had known
B. had I known
C. I knew
(答案→)D. did I know
解析：句意：只有当我设身处地为他们想的时候，我才知道，他们比我更加痛苦。only＋状语从句位于句首时，主句要用部分倒装，know这个动作发生在put之后，所以不能用过去完成时，因此选D。