Only when he started to explain ____________ the reason for this.

问题描述:

Only when he started to explain ____________ the reason for this.
A.she realized B.did she realize C.she had realized D.had she
用的是什么语法?
D.had she realized 刚刚打漏了……

Only when he started to explain had she realized the reason for this.
选项D是正确的,从句是一般过去时,主句用过去完成时.only在句首时主句要用倒装形式.
翻译:只有他开始解释时,他才真正明白了这件事的原由了.