英语虚拟语气的大的原则是“时态倒推法”,如下：
与现在事实相反谓语动词用过去时,如例句所示：If I knew her telephone number,I would call her.
If I were a bird,I would fly aroud the world.
与将来事实相反用三种表示法：谓语动词用过去时；should do；were to do.
例句：If he came/should come/were to come tomorrow,I would go to the airport to meet him.
与过去事实相反谓语动词用过去完成时,例句如下：
If I had read that book,I would be able to answer the teacher's question now.