问题补充:
Not until he failed the exam he had left his books untouched .
A. did he realize
B. had he realized
C. he realized
D. he had realized
答案:
(答案→)A. did he realize
B. had he realized
C. he realized
D. he had realized
解析:句意:直到他考试失败了,他才意识到他没有碰书本。Not until+时间状语从句,放在句首的时候,后面的主句用部分倒装,助动词放在谓语前面,因为realize是发生在fail后面,不是前面,不能用过去完成时,用一般过去式,选A。