Not until he failed the exam he had left his books untouched .A. did he realizeB. had he r

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。