Question

Only when class began_____that he had left his book at home.A.will he realizedB.he did realize

C.did he realizeD.should he realize
为什么要用did he realize如果不是倒装那语序应该是he did realize,感觉好奇怪

Answer 1

这是一个由only引导的倒装句，所以这里用did he realize。
在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。但如果only后跟
的是主语，就不用倒装结构了。

倒装句就是将正常的陈述语序加以变化，主要作用是强调被提前的部分。
倒装句可分为全部倒装和部分倒装两种：
一、全部倒装
谓语动词提前至主语之前为全部倒装，如：
Out came his guest.
On the hill stood a little cottage that contained heaps of hay.
There lived an elderly lady whose husband died long ago.
二、部分倒装
助动词或情态动词提至主语之前，谓语动词位置不变，即为部分倒装，如：
Neither could he see through your plan.
So little did I know about him that I was easily taken in by his words.
Doesn't her invitation appeal to you?
第一节 否定词提前倒装
否定词用于句首时，句子应进行部分倒装
否定词常用的有：
Not only…(but also), 　　　　　　　Not until(直到…..才),
No sooner….(than)(一…..就) Never/ Rarely/Seldom
Hardly/Scarcely… (when) Few/Little
Neither/Nor (也不)　　　　　　　 Nwhere
At no time　　　　　　　 Under no circumstances(决不)
On no account (决不)　　　　　　　In no way
其中not only, no sooner, hardly, scarcely 分别和but (also), than, when搭配使用，需注意：后一组词之后的部分不进行倒装，只有否定词之后的部分倒装
如：Hardly had I arrived home when it began to rain.
Not only was he able to enter the final round of the contest, but he came out first as well.
No sooner had I got any chance to speak than the clerk slammed the door in my face.
Seldom does he travel about.
Under no circumstances should you betray your own country.
此类倒装句一般出现在填空题中，需填入的是倒装部分，只要在句首见到上述的词，就应条件反射的想到用倒装句。

希望对您有帮助，满意请采纳，谢谢！O(∩_∩)O

Answer 2

only+状语放句首，部分倒装
部分倒装就是用疑问句的形式来倒装
he realized......陈述句
did he realize.....一般疑问句
就是疑问句形式

Answer 3

用 did 吧