if的条件从句分为真实条件从句和虚拟条件从句.而真实条件从句适用于主将从现,是假设有可能发生的.而虚拟条件从句是假设不太可能发生的.

　　IF的虚拟语气有三种：

　　内容从句主句

　　与现在事实相反动词过去式（be用were）would/could/should/might+动原

　　与过去事实相反had+动词过去分词would/could/should/might+have+动分

　　与将来事实相反should/wereto+动原would/could/should/might+动原

　　下面三句依次是上面三种情况的例句：

　　Ifiwereyou,Iwould/could/should/mightdothat.

　　Ifihadknowthatyesterday,Iwould/could/should/mighthavetoldyou.

　　Ifitshouldrain,thecropswould/could/mightbesaved.