only when从句后面的主句为什么不用倒装

问题描述

在[美]克里斯汀、[美]金姆 和李文昊著的《英语语法看这本就够了大全集》的第333页上有这样一个句子：
It is only when one is ill that one realizes the importance of health. 人们只有在生病时才知道健康的重要性。
英语部分为什么不是：It is only when one is ill that does one realize the importance of health.

老师答疑

刘老师：

这不奇怪。这句话原本是：
1. One realizes the importance of health only when one is ill.
现在，有两种强调方式：
第一种，把only when one is ill 提到句首。
第二种，用It is强调only when one is ill。
2.  Only when one is ill does one realize the importance of health.
【注】在句首的是only，符合主句倒装条件。
3.  It is only when one is ill that one realizes the importance of health.
【注】在句首的是It，而不是only，无法满足倒装条件。
这跟 not... until...是一回事。例如：
1. He didn’t realize his mistake until the teacher had told him about it.
2. Not until the teacher had told him about it did he realize his mistake.
3. It wasn’t until the teacher had told him about it that he realized his mistake.
以上两组句子，只有句2 能够满足倒装的条件：only和not在句首。
若有不足或错误，请黎老师及各位批评指正。