Only when I put myself in their position they were more miserable than me.A. I had known　

问题补充：

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

答案：

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。