虛擬語氣用來表示假想，而不表示客觀存在的事實，所說的是一個條件，不一定是事實，或與事實相反。
虛擬語氣通過謂語動詞的特殊形式來表示。英語中的語氣分為陳述語氣、祈使語氣、虛擬語氣三類。
虛擬語氣表示說話者做出的假設并非事實，或難以實現的，甚至表達相反的概念。另外表示主觀愿望或表達某種強烈的感情時，也用虛擬語氣。
一、虛擬語氣用于假設
略。（參見條件和假設章節）
二、虛擬語氣用于以as if和as though（好像）引導的方式狀語從句中，表示不太符合實際的比喻。
This minicomputer operates as if it were a new one.
This microcomputer operates as though it had been reqaired.
注意：在look和其他個別動詞后引起的as if 和as though的從句內，可以用虛擬語氣，目前有趨勢，也可以用陳述語氣，因為as if和as though（作為“好像”）來講已經襯托出虛擬語氣的含義。
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、虛擬語氣用于lest， for fear that 和 in case引導的目的狀語從句中。
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：以上三個目的狀語從句內的should還可以省略，于是就以動詞原形出現。
四、虛擬語氣用于though(或although), even if和even though引出的讓步狀語從句中，表示已不把讓步作為事實，而是把它作為假設來用。
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.
上句是用動詞原形表示虛擬語氣，借以體現不把讓步當作事實。于此同理，如果在條件從句內也采用動詞原形表示虛擬語氣，當然也可以把條件轉為假設。例如：
If the news be true, his life is in the greatest danger.
五、虛擬語氣用于表示愿望的固定句型結構中
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、虛擬語氣用于主句中含有建議、勸告、要求、請求、需要、愿望、命令等詞語的有關從句中，體現一種“讓某人做某事”的主觀意向
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：屬于本類用法的詞語還有：demand, propose, command, request, insist, suggestion, recommendation, motion(作”動作“講），plan等。見到這些詞，后面從句用should或用動詞原形，便是虛擬語氣的用法了。
七、虛擬語氣用于表示祝愿的祈使句中，一般用動詞原形或may加動詞原形
Long live the friendship between the peoples all over the world.
May your life be long.
八、虛擬語氣用于表示主觀揣測和婉轉的語氣
At that time the temperature would rise rapidly.
Would you write a letter to him?
Might I come in?
九、虛擬語氣用于it is time后的定語從句
It is (high) time (that) we (should) set off.
It is time (that) we got up.
十、虛擬語氣用于表示驚奇、疑惑、沒料到的事
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：對過去的事情表示驚奇，should后面用完成式動詞；對今后的事情表示驚奇，則should后面用一般式動詞。
十一、虛擬語氣用于it is important（或natural), it has been dicided, I am sorry, they were disappointed 等句子后面的從句中
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等用于虛擬語氣和作其他用法時的區別。
詳見should, would, might各自的用法的專題介紹。
