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/were to+动原 would/could/should/might+动原 下面三句依次是上面三种情况的例句: If i were you ,I would/could/should/might do that. If i had know that yesterday,I would/could/should/might have told you . If it should rain ,the crops would/could/might be saved.