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

问题描述：

only when classbegan____thathe had left his book at homeA.will realize B.he did realizeC.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。

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最佳答案：

选C，Only when class began did he realize that he had left his book at home．

句意：只有当开始上课时，他才意识到他把书丢在了家里。

解析：结合语境可知本句描述的是过去发生的动作，故用过去时态。当"only+状语"位于句首，主句用部分倒装，助动词提前。注意：当only修饰主语放句首时，不倒装。

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