only when class began----that he had left his book at home.

only when classbegan____thathe had left his book at home A.will realize B.he did realize C.did he realize D.should he realize 为什么D选项不正确，还有怎么看它是一般过去时，C.选项如果是一般过去时那么realize不是＋d吗，求帮解答一下整个题目。