英語(yǔ)虛擬語(yǔ)氣的大的原則是“時(shí)態(tài)倒推法”,如下：
　　與現(xiàn)在事實(shí)相反謂語(yǔ)動(dòng)詞用過去時(shí),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.
　　與將來事實(shí)相反用三種表示法：謂語(yǔ)動(dòng)詞用過去時(shí)；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.
　　與過去事實(shí)相反謂語(yǔ)動(dòng)詞用過去完成時(shí),例句如下：
　　If I had read that book,I would be able to answer the teacher's question now.