与现在事实相反:从句谓语:did/were,主句谓语:would/should/could/might+do
与过去事实相反:从句谓语:had done,主句谓语:would/shoud/could/might+have done
与将来事实相反:从句谓语:did/were,were to do,should do,主句谓语:四情+do
常考省略if的虚拟
3.将were/had/should提到主语前(否定词不提前)
eg:If you hadn't helped me ,I wouldn't have 。Had you not helped me