虛擬語氣表示說話人的主觀願望、猜疑、建議或與事實不符的假設等，而不表示客觀存在的事實。虛擬語氣是由句中的謂語動詞的特殊形式表示出來的。
虛擬語氣主要分為與現在事實相反，與過去事實相反以及與將來事實相反三個種類。讓我們一一來看下它們的結構和用法吧！
1、表示與現在事實相反的情況
從句：If+主語+動詞一般過去時(Be動詞用were)
主句：主語+ should/would/might/could+do：
eg：If I were you，I would take an umbrella.
如果我是你，我會帶把傘。（事實：我不可能是你）
If I knew his telephone number，I would tell you.
如果我知道他的電話號碼，我就會告訴你。（事實：不知道）
If I had any money with me，I could lend you some.
如果我帶錢了，我就會借給你些。（事實：沒有帶錢）
2、表示與過去事實相反的情況
從句：If+主語+had+done
主句：主語+should/would/might/could+have done
eg：If I had got there earlier，I should/would have met her.
如果我早到那兒，我就會見到她。（事實：去晚了）
If he had taken my advice，he would not have made such a mistake.
如果他聽我的勸告的話，就不會犯這樣的錯誤了。
（事實：沒有聽我的話）
3、表示與將來事實相反
從句：①if+主語+were to do
②if+主語+should+do
③if+主語+動詞一般過去式（be動詞用were）
主句：主語+should/would/might/could+do
eg：If she were to be here next Monday，I would tell her about the matter.
如果她下周一來這兒的話，我就會告訴她這件事的始末。（事實：來的可能性很小，不可能）
If he should come here tomorrow，I would talk to him.
如果他明天來這兒的話，我就跟他談談。（事實：來的可能性很小，不可能）
If there were a heavy snow next Sunday，we would go skating.
如果下周日下大雪，我們就去滑冰。（事實：下雪可能性很小，不可能）
除了以上三種主要情況，虛擬語氣還有以下幾種結構和用法哦。
1、 當從句的謂語動詞含有were，should，had時，if可以省略，這時從句要用倒裝語序，即把were，should，had等詞置於句首，這種多用於書面語。
eg：Should he agree to go there，we would send him there.
要是他答應去的話，我們就派他去了。
Were she here，she would agree with us.
如果她在這兒的話，她會同意我們的。
Had he learnt about computers，we would have hired him to work here.
如果他懂一些電腦知識的話，我們已經聘用他來這裡工作了。
2、非真實條件句中的條件從句有時不表達出來，只暗含在副詞、介詞短語、上下文或其他方式表示出來，這種句子叫做含蓄條件句，在多數情況下，條件會暗含在短語中，如without…，but for…（要不是因為...）等
eg：
But for his help，we would be working now.
要不是他的幫助，我們還會在工作呢。
Without your instruction，I would not have made such great progress.
要是沒有你的指導，我不會取得如此大的進步。
We didn't know his telephone number，otherwise we would have telephoned him.
我們不知道他的電話號碼，否則我們就會給他打電話。
3、 有時，虛擬條件語氣的從句中，主、從句可以省略其中的一個，來表示說話人的一種強烈的感情。
①省略從句
He would have finished it.
他本該完成了。
You could have passed this exam.
你本能通過這次考試的。
②省略主句
If I were at home now.
要是我現在在家裡該多好啊。
If only I had got it.
我要是得到它了該多好啊。
4.注意，在虛擬語氣的從句中，動詞be的過去時態一律用were，不用was。
If I were you，I would go to look for him.
如果我是你，就會去找他。