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：IfIknewhertelephonenumber,Iwouldcallher.

　　IfIwereabird,Iwouldflyaroudtheworld.

　　与将来事实相反用三种表示法：谓语动词用过去时；shoulddo；weretodo.

　　例句：Ifhecame/shouldcome/weretocometomorrow,Iwouldgototheairporttomeethim.

　　与过去事实相反谓语动词用过去完成时,例句如下：

　　IfIhadreadthatbook,Iwouldbeabletoanswertheteacher'squestionnow.