英語文法中有三種不同的語氣。陳述語氣用于提出問題、陳述事實或表達觀點；祈使語氣用于訓示，請求，希望；而虛拟語氣表達願望，要求，建議或假設。
一、結構
1.與現在的事實相反
從句用過去式，主句would/could/might/should/+動詞原形。
If I had time ，the classroom would be cleaner.
如果我有時間，教室将會更加幹淨。
2.與過去的事實相反
主句would/could/should/might/+have+ done , 從句中過去完成時。
If I had finished my homework ，I would have passed that test.
如果當時我完成了我的作業，我就能夠通過我的考試了。
3.與将來的事實相反
主句用should/could/would/might/+動詞原形，從句用should do （可能性最大），過去式（可能性一般）were to do （可能性最小）。
If it should snow tomorrow，I would make a snowman.
If it snowed tomorrow，I would make a snowman.
If it were to snow tomorrow ，I would make a snowman.
如果明天下雪，我将堆一個雪人。
二、虛拟語氣六大句型
1.was/were going to 過去原本要
I was going to go shopping, but I got a phone call which lasted nearly an hour, and by the time the call ended the shops were all shut.
我原本要去逛街，但我接到了一通通話時間将近一小時的電話，而當通話終于結束時，所有店家都關門了。
2.was/were set to 過去本來準備（做）
I was set to go on holiday, but then my car broke down, and I had to spend the money on repairs instead.
我原本準備好要去度假了，可是我的車壞掉了，而我隻能花錢去修理它。
3.would have 本來想要
I would have called you yesterday but my phone wasn’t working.
我昨天本來要打給你的，但我的電話壞掉了。
4.had the intention of 原本有做某事的意圖
I had the intention of returning the book to the library last week, but I have a friend staying, and she’s been reading it.
我上禮拜原本要把書還回圖書館的，但我有個朋友暫住下來，而她還在讀那本書。
5. had it all arranged to 原本安排好（做）
I had it all arranged to give my friend a surprise party, but then she found out about my plan, and said she didn’t want one!
我原本預計要給我朋友一個驚喜的派對，但她發現了我的計劃，而且說她不想要！
6. if only 如果、要是
If only I’d listened to your advice, then I wouldn’t have invested in that stock. I’ve lost a lot of money.
如果我當初有聽你的意見，我就不會投資那支股票了。我賠了很多錢。
三、虛拟語氣用于名詞性從句
1.虛拟語氣在賓語從句中的運用
（1）“wish + 賓語從句”表示不能實作的願望，譯為“要是……就好了”等。表示現在不能實作的願望，從句中的謂語動詞用一般過去時；表示将來不能實作的願望，從句中的謂語動詞用“would/could + 動詞原形”；表示過去不能實作的願望，從句中的謂語動詞用“had + 過去分詞”或“could(should) + have + 過去分詞”。
I wish it were spring all the year round.
要是全年都是春天就好了。
I wish I had known the answer.
要是我知道答案就好了。
I wish I could fly like a bird.
要是我能像鳥兒一樣飛翔就好了。
（2）在表示建議、要求、指令等的動詞suggest，order，advise，propose，demand，require，insist request，command等後的賓語從句中，謂語動詞用should + 動詞原形或是動詞原形。
She suggested we (should) leave here at once.
她建議我們最好立刻離開這兒。
The doctor ordered she should be operated.
醫生指令給她動手術。
2.虛拟語氣在同位語從句和表語從句中的運用
作表示建議、要求、指令等的名詞advise、idea、order、demand、plan、proposal、suggestion、request等的表語從句和同位語從句，從句中的謂語動詞用“（should） + 動詞原形”。
His suggestion that we (should) go to Shanghai is wonderful.
他提出的要我們去上海的建議真不錯。
My idea is that they (should) pay 100 dollars.
我想他們應該付100美元。
3.虛拟語氣在主語從句中的運用
（1）在主語從句中，謂語動詞的虛拟語氣用“should + 動詞原形”的結構，表示驚奇、不相信、理應如此等。
It is necessary that we should clean the room every day.
我們每天收拾屋子是很必要的。
It was a pity that you should be so careless.
太遺憾了，你這麼粗心。
It will be desired that she should finish her homework this afternoon.
希望她今天下午完成她的作業。
（2）從句表示的是事實時，如果說人對這種事實表現出驚奇的情感，就可用虛拟語氣。反之，如果不表示驚奇等情感，that從句也可用陳述句。
It is pity that you can’t swim.
你不會遊泳真令人遺憾。
四、情态動詞表示的虛拟語氣
1.should +動詞原形（有時省略should）
（1）用在動詞如advise，command，demand，desire，insist，order，propose，recommend，request，urge，require，suggest等後的賓語從句中。
They requested that we send a delegation to their country.
他們要求我們派一位代表去他們國家。
（2）用在it is suggested，it is desired，it is required，it was ordered，it was proposed，it has been decided，it is necessary that等引導的主語從句中。
It is desired that we (should) get everything ready by tonight.
我們希望今晚把一切準備妥當。
（3）用在suggestion，motion，proposal，order，plan，idea，recommendation，requirement等引導的表語從句和同位語從句。
He put his coat over the child in case he should catch cold.
他把外套蓋在孩子身上以免他感冒。
2.用在表示比拟的方式狀語從句中
They talked as if they had been friends for years.
他們交談甚歡，就好像是熟識多年的朋友一樣。
3.用在表示虛拟情況的定語從句中
It is time you handed in your test paper.
該是你交卷紙的時候了。
4.用在某些表示主觀願望的名詞從句中
I wish I were as strong as you.
我希望我像你一樣強壯。
He insisted that we take up the matter at the meeting.
他堅持要我們在會議上讨論這件事。
5.用在婉轉的請求、建議、批評等句子中
Could you spare me a few minutes?
你能抽出幾分鐘時間嗎？
You might have told me earlier.
你本可以更早點告訴我的。
Hadn’t you better go and see you dentist about that tooth?
關于你那副牙，你不覺得最好去看牙醫嗎？
6.用在表示祝願的簡單句中
Long live world peace.
願世界長久和平。
May you succeed.
你會成功的。
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