对过去的事情进行虚拟语气。
could/would/should/might + have done +if sb + had done .
对现在事情进行虚拟语气。
could/would/should/might + do + if sb + did/were
对将来事情进行虚拟语气。
could/would/should/might + do + if sb + did/were
could/would/should/might + do + if sb + should do
could/would/should/might + do + if sb + were to .
因为 do + if sb + were to。只是套用公式。
所以也没什么可不可以理解的。就是套用公式就行。
无论是什么主语,遇到虚拟语气后面都要用were修饰就可以了、
参考资料:上课的小笔记而已。