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
网友回答
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。