虚拟语气用来表示假想，而不表示客观存在的事实，所说的是一个条件，不一定是事实，或与事实相反。
虚拟语气通过谓语动词的特殊形式来表示。英语中的语气分为陈述语气、祈使语气、虚拟语气三类。
虚拟语气表示说话者做出的假设并非事实，或难以实现的，甚至表达相反的概念。另外表示主观愿望或表达某种强烈的感情时，也用虚拟语气。
一、虚拟语气用于假设
略。（参见条件和假设章节）
二、虚拟语气用于以as if和as though（好像）引导的方式状语从句中，表示不太符合实际的比喻。
This minicomputer operates as if it were a new one.
This microcomputer operates as though it had been reqaired.
注意：在look和其他个别动词后引起的as if 和as though的从句内，可以用虚拟语气，目前有趋势，也可以用陈述语气，因为as if和as though（作为“好像”）来讲已经衬托出虚拟语气的含义。
She looks as if she is from Beijine.
It seems as if we we shall have to walk home.
Is sounds as though someone is knocking at the door.
三、虚拟语气用于lest， for fear that 和 in case引导的目的状语从句中。
He is working hard for fear that he should fall behind.
I wrote everything down lest I should forget it.
Take your umbrella along, in case it should rain.
注意：以上三个目的状语从句内的should还可以省略，于是就以动词原形出现。
四、虚拟语气用于though(或although), even if和even though引出的让步状语从句中，表示已不把让步作为事实，而是把它作为假设来用。
Even though she had been very busy, she would have helped us.
Even if he were here, he could not solve the problem at once, eighar.
Though (Although) he blame me, yet will I trust in him.
上句是用动词原形表示虚拟语气，借以体现不把让步当作事实。于此同理，如果在条件从句内也采用动词原形表示虚拟语气，当然也可以把条件转为假设。例如：
If the news be true, his life is in the greatest danger.
五、虚拟语气用于表示愿望的固定句型结构中
You had better clear up this problem.
We would like to see her.
Lisa would sooner go downtown than stay here.
I had (would) rather take his than that.
He would rather you didn't do that for the time being.
Prof. Lu would as soon get online as go to the cinema.
六、虚拟语气用于主句中含有建议、劝告、要求、请求、需要、愿望、命令等词语的有关从句中，体现一种“让某人做某事”的主观意向
They suggest that the World Cup be held every two years.
I recommended that he (should) study more English before going aboard.
they require that this work (should) be done at once.
She thinks it necessary that we should obtain experience through practice.
His idea is that we (should) send a teacher to teach them English.
I agree to the proposal that we (should) put this question to the vote.
It is desired that you (should) put yourself in his position.
It will be better that we (should) hold a meeting next week.
He ordered that the soldiers should put out the lights immediately.
注意：属于本类用法的词语还有：demand, propose, command, request, insist, suggestion, recommendation, motion(作”动作“讲），plan等。见到这些词，后面从句用should或用动词原形，便是虚拟语气的用法了。
七、虚拟语气用于表示祝愿的祈使句中，一般用动词原形或may加动词原形
Long live the friendship between the peoples all over the world.
May your life be long.
八、虚拟语气用于表示主观揣测和婉转的语气
At that time the temperature would rise rapidly.
Would you write a letter to him?
Might I come in?
九、虚拟语气用于it is time后的定语从句
It is (high) time (that) we (should) set off.
It is time (that) we got up.
十、虚拟语气用于表示惊奇、疑惑、没料到的事
It is strange that she should have failed to do this test.
It is wonderful that she should have completed the experiment in a short time.
I never thought that the problem should be so easy.
We just couldn't believe that the new building should be so beautiful.
注意：对过去的事情表示惊奇，should后面用完成式动词；对今后的事情表示惊奇，则should后面用一般式动词。
十一、虚拟语气用于it is important（或natural), it has been dicided, I am sorry, they were disappointed 等句子后面的从句中
It is very important that China should join WTO.
It is natural that I should drop in on my teather.
It has been decided that we should have English classes next week.
I am sorry that He should be so weak.
十二、注意should, would和might等用于虚拟语气和作其他用法时的区别。
详见should, would, might各自的用法的专题介绍。
