it is only when.that.强调句

it is only when.that.强调句
例如It is only when we cannot see perfectly that we realize how important our eyes are.强调句的话应该去掉it is和that之后句子还应该完整.那么剩下的only when引导的时间状语从句only打头修饰状语部分,后面为什么不倒装呢?（就是为什么不用that do we realize how important our eyes

Only when在主从复合句句首主句要倒装的情况
倒装的条件：
1）only在句首
2）句子为主从复合句
It is only when we cannot see perfectly that we realize how important our eyes are.
该强调句Only不在句首,也不是主从复合句,所以不满足以上条件,不需倒装.
强调句若去掉it is和that之后句子是应该完整的.只是该强调句有点特殊,因为only出现在句首了,所以主句应该倒装.
Only when we cannot see perfectly do we realize how important our eyes are.