1、在表示虚假的，与事实相反的或难以实现的情况时用虚拟语气；表示主观愿望的或情感时，也用虚拟语气。
2、与现在事实相反的虚拟语气。常用结构为：条件句的谓语动词为were或动词的过去式，主句谓语动词为“would/should/could/might+动词原形”。例如lf l were ten years younger,l would start all over again.lf l had enough money,l would first of all buy myself a new comupter.
3、与过去事实相反的虚拟语气。常用结构为：从句用“had+过去分词”，主句用“would/should/could/might+have done”。例如：lf we had had enough rain last year,we could have gained a good harvest.
4、与将来事实相反的虚拟语气。常用结构为：从句谓语可用动词过去式，“should+动词原形”或“were to+动词原形”来表示。例如：lf l should see/were to see/saw him tomorrow,l would invite him home.
5、省略if的虚拟语气。if虚拟条件句中有should，had或were时，if可以省略，从句中主谓要倒装。例如：lf l had worked hard,l would have surely succeeded.Hadlworked hard,l would have surely succeeded.
6、混合虚拟语气。有时主句从句所指的时间不一样，这叫混合虚拟语气。它要求主从句跟据实际时间概念确定。例如：lf she had followed the doctor`s advice ,she would be quite all right now.Had you told me the news yesterday ,l would not be so worried now.
7、含蓄虚拟语气。有时假设的条件不通过条件从句表达，而隐藏在介词短语中，如：without、but for、 otherwise 、or等。例如：But for your advice ,l would have failed.=lf you had not given me advice ,l would have failed.
