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.