Only when class began that he had left his book at home．
A. will he realize
B. he did realize
C. did he realize
D. had he realized
网友回答
A. will he realize
B. he did realize
(答案→)C. did he realize
D. had he realized
解析：考查时态及主谓倒装，句意：只有当开始上课时，他才意识到他把书丢在了家里。结合语境可知本句描述的是过去发生的动作，故用过去时态。当only位于句首时，主句用部分倒装，助动词提前。故选C。