if 的虚拟语气

一．定义

虚拟语气是一种特殊的动词形式，用来表示说话人所说的话并不是事实，而是一种假设，愿望怀疑或推测。虚拟语气又称作非真实条件句。

二．构成

if+主语+动词过去式，主语+would/should/could/might+动词原形(即:条件从句，主句)

三．用法

1.表示与过去事实相反的假设，if从句的谓语形式用过去完成时，即had+过去分词,主句用would/could /might+have+过去分词

If I had done it，I should have told you.如果我做了,我应该就会告诉你。

2.表示与现在事实相反的假设,if从句的谓语形式用动词的过去式(be动词一般用were),主句用would/should/could/might+动词原形

If she were you，she might help me.如果她是你，她可能会帮我。

3. 表示对将来不确定的假设，if从句的谓语形式用一般过去式或were to /should+动词原形,主句用would/should/could/ might+动词原形。

If he were to go home today,he would not come here tomorrow.如果他今天回家，他明天就不会来这儿。

4. 可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if省去，而将had ，should， were等词提到主语之前，即用倒装结构。

Had she studied harder,she would have passed the exams.(=If she had studied harder，she would have passed the exams.)如果她更努力学习她就会通过考试了。

5.有时虚拟条件句并没有用if从句表示出来，而是用介词短语(otherwise， or ，without，but for)、上下文或其他方式来表示。

Without your help，I would lose my job.没有你的帮助我会失业。

6.有时表示虚拟语气的条件从句或者主句都可以省略，而只剩下一个主句或者一个条件从句。

I could have told you.我本来可以告诉你的。

If I were young.我要是年轻该多好啊！