Only when he had done it that he had made a mistake.A. he then realizedB. did he realizeC

问题补充：

Only when he had done it that he had made a mistake.

A. he then realized

B. did he realize

C. before

D. he realized

答案：

A. he then realized

(答案→)B. did he realize

C. before

D. he realized

解析： 考查倒装，only + 状语开头句子采用倒装。 句意：只有当他做了那件事后他才意识到他犯错误了。