Not until he failed the exam__that working hard is important.
A> had he known B>did he know C>he knew D>he had known
选哪个,为什么 working hard is ...现在时
答案解析：
Not until 主语+谓语 +倒装结构（助动词提前）+主语+ 谓语 结构（主句倒装,从句不倒装）
Not until he failed the exam did he know that working hard is important.
直到考试失败之后他才知道努力学习是很重要的