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. should he realize
网友回答
A. will he realize
B. he did realize
(答案→)C. did he realize
D. should he realize
解析：考查倒装结构。当only等词谓语句首时，主句采用部分倒装，句意：直到上课开始时，他才意识到他已经把书丢在家里面了。结合语境可知此处描述的是过去的动作，故用过去时态，助动词用did，故选C。