【题目】Only when class began ______ that he had left his book at home.
A. will he realizeB. he did realize
C. did he realizeD. should he realize
在线课程【答案】C
【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。