Only when he got on the train that he had left his mobile phone at homeA. he realizedB.

问题补充：

Only when he got on the train that he had left his mobile phone at home．

A. he realized

B. he had realized

C. had he realized

D. did he realize

答案：

A. he realized

B. he had realized

C. had he realized

(答案→)D. did he realize

解析：考查：only+状语放在句首，使用部分倒装。根据时态所以选D。