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如果從語氣的角度進行分類，英語的所有句子可分為三種，分別是陳述語氣、祈使語氣以及虛擬語氣。
不論什麼時候，我們說話都是帶著個人情感的，有時候我們只是客觀地陳述事實，從語法角度研究，就是陳述語氣：
The Earth travels around the Sun.
地球繞著太陽轉。
He has been working in the company for twenty years.
他在公司幹了二十年了。
Most Chinese parents push their children too hard.
大多數中國父母把孩子逼的太狠了。
有時候，我們向對方發出請求、警告，建議或命令，可以用祈使語氣：
Please turn off the TV.
請把電視關了。
Don't smoke in the office.
不要在辦公室抽菸。
Stop complaining, will you?
別怨天尤人的，好嗎？
Would you be so kind as to lend me a hand?
請幫個忙好嗎？
那麼，我們什麼時候會用虛擬語氣？
首先，請牢記，虛擬語氣是通過謂語動詞的變化表達的：
If I were you, I would not believe his story.
如果我是你，我不會相信他的故事。
一、表示說話人的主觀願望：
1，wish表示願望，有三種情況：
1）表達對未來的期望，不管可不可能，賓語從句的謂語用would/could+動詞原形：
I wish you could join us tomorrow.
我希望你明天能加入我們。
I just wish I could go home.
我只希望我能回家。
I wish there would be no famine any more in the world.
我希望世界上不再有饑荒。
2）表達與現在事實相反的願望，從句謂語用一般過去時或過去進行時:
She wishes she had a little brother.
她希望她有一個弟弟。
I wish we were living in the countryside.
我希望我們住在鄉下。
They wished it were/was summer.
他們希望是夏天。
注意：傳統上，虛擬語氣裡，主語是單數時，系動詞be要變were，現代英語裡也可以用was。
3）表達與過去事實相反的願望，從句謂語用過去完成時或would / could + have + done:
I wish you hadn't done that.
我希望你沒有那樣做。
（事實是，那樣做了）
How she wished she had not turned him down.
她多麼希望自己沒有拒絕他。（事實是，她拒絕了）
I wish I had not left school when I was sixteen.
我希望我16歲的時候沒有輟學。（事實是，我輟學了）
I wish I could have come to last night's party.
我真希望我能來參加昨晚的聚會。（事實是，沒能參加）
2，It's (high) time that...,從句謂語用過去時
It's time you went to bed. It's Monday tomorrow.
你該睡覺了。明天是星期一。
It's about time we started to save for college tuition.
我們該開始攢錢上大學了。
It's high time that all of us fought against drug abuse.
現在是我們所有人與吸毒做鬥爭的時候了。
3，would rather寧願，從句謂語用過去時
I』d rather you stayed with your father tonight.
我寧願你今晚陪陪你父親。
He'd rather that you called the police.
他寧願你報警。
4，一些祝願，願望等：
If only I were eighteen.
我要是十八歲就好了。
May you always be happy!
願你永遠幸福！
May God be with you all！
願上帝與你們同在！
Heaven forbid！
但願不會這樣！
二、表示說話人的命令，要求或建議
1，以下表示命令，要求或建議的動詞或名詞後從句謂語用should + do（如果謂語是動詞原形，那是說話人省略了should）：
demand要求, insist堅持, order命令, require要求, suggest建議，suggestion建議等
The manager demanded that she leave for Beijing right away.
經理要求她馬上動身去北京。（省略了should）
She insisted that she go to college alone.
她堅持要一個人去上大學。
An order was given that we cross the river before dawn.
下達了要求我們黎明前過河的命令。
（that從句是名詞order的同位語從句）
The teacher ordered that we all go out of the classroom.
老師下令我們全體離開教室。
Schools require that each student be on time daily.
學校要求每個學生每天準時上課。
Father suggested that we go to the countryside this weekend.
父親建議我們這個周末去鄉下。
My suggestion is that a meeting should be held to discuss it.
我建議開個會來討論這個問題。
請注意：insist表示「堅持認為」，suggest表示「暗示，表明」時，應該是陳述句形式。
2，It is best最好/crucial至關重要的/desirable值得做的/essential完全必要的/important重要的/ urgent迫切的/vital極重要的/necessary必要的，that從句謂語也是（should）do
It's desire that a woman (should) be equally paid for her equal work with men.
女人與男人同工同酬是我們的願望。
It's important that he (should) recognize his strengths and weaknesses.
他必須認識到自己的長處和短處。
It's crucial that the habitats of giant pandas (should) be well protected.
保護好大熊貓的棲息地是至關重要的。
三、表示說話人的猜測、或與事實相反的的假設：
1，情態動詞表示猜測或與事實相反
表示猜測或與事實相反是情態動詞除表示情感態度之外另一主要用法：
He must have passed his exam. He looks happy.
他一定通過了考試。他看起來很高興。
（對過去事實的猜測）
They must be at home. the lights are on.
他們一定在家。燈都亮了。
（對現在的猜測）
They must have goneout. the lights are off.
他們一定出去了。燈都沒亮。
（對過去事實的猜測）
You should have told me about your trouble.
你本該告訴我你的麻煩。
（與事實相反）
2，As if/though 似乎，好像
As if/though…後可能是陳述句，如果表達的是與事實相反的情況，要用虛擬語氣。
It looks as if you've met before?
你們好像見過面？
（陳述語氣）
It looks as if it's going to rain.
好像要下雨了。
（陳述語氣）
It seemed as if the lecture would never end.
這堂課似乎永遠不會結束。
（與事實相反）
They felt as if they hadn't had anything for a week.
他們覺得好像一個星期沒吃東西了。
（與事實相反）
3，虛擬條件句
條件狀語從句也可能是陳述句，當表達的是對未來的假設，或與過去，現在的事實相反的情況，要用虛擬語氣。
1）對未來的假設（未來發生可能性不大）：
從句的謂語用過去式；也可用should + do，主句謂語用「should (would, could, might)+do」：
If a war broke out, millions of lives could die because of the nuclear weapons.
如果戰爭爆發，數百萬人可能因核武器而死亡。
（可能性小）
If I were to be a rich man, I would make more charitable donations.
如果我成為有錢人，我會為慈善捐更多款。
If I should become the president, I would build more universities.
如果我成為總統，我會建更多的大學。
（可能性小）
2）與現在的事實相反:
從句的謂語用過去式，主句謂語用「should (would, could, might)+do」：
If he were here right now, he could tell us what to do next.
如果他在這裡，他可以告訴我們下一步怎麼做。
If there were an elevator in the building, life for the olds could be more convenient.
如果樓裡有電梯，老年人的生活會更方便。
If I were you, I would marry her immediately.
如果我是你，我會立刻娶她。
If I knew where she lives, I might visit her this afternoon.
如果我知道她住在哪裡，我今天下午可能會去看她。
3）與過去事實相反:
從句謂語用「had+過去分詞」，主句謂語用「should (would, could, might)+have+done」：
If you had taken a taxi, you wouldn't have missed the train.
如果你坐計程車的話，你就不會錯過火車了。
If you had worked harder, you couldn't have failed any of your finals.
如果你努力學習，你的期末考試就不會有掛科。
If he hadn't driven after he was drunken, he couldn't have killed two people.
如果他沒有醉酒開車，就不可能撞死兩個人。
I should have already finished my term paper if I hadn't caught a cold.
如果沒有感冒的話，我應該已經完成我的學期論文。
注意：
1）從句中有should, were, had done時，為了簡練，可以用倒裝句，就是省略if，把should, were, had提前到主語前面：
Should it snow, the road in the mountains could be blocked.
如果下雪，山上的路可能會被堵住。
Were it to snow, we would not set off to the mountains.
如果下雪，我們就不去山上了。
Had he listened to his doctor, he would not have suffered such a pain.
如果他聽了醫生的話，他就不會受這種痛苦了。
2）虛擬條件句中，有時候主從句的時間可能不一致，在使用時我們要注意謂語變化的不同：
If I had studied physics at university, I could be a famous physicist now.
如果我在大學裡學過物理，我現在可能是一個著名的物理學家了。
（從句與過去事實相反，主句與現在事實相反）
If I had taken the money, I would feel guilty for the rest of my life.
如果我拿走了錢，我會愧疚終生。
（從句與過去事實相反，主句與現在事實相反）
If he were not in hospital, he would have gone to Beijing.
如果沒有生病住院，他早就去北京了。
（從句與現在事實相反，主句與過去事實相反）
3, Without+名詞，相當於條件從句，句子有時要用虛擬語氣：
Without water, there would be no life on Earth.
沒有水，地球上就沒有生命。
Without your help, I could have died in the desert.
要是沒有你的幫助，我可能會死在沙漠裡。
四、目的狀語從句中的虛擬語氣
for fear that, in case, lest目的狀語從句中可以用虛擬語氣；lest用的最多。可用可不用的我們可以就用陳述句。
如果大家要用虛擬語氣時，從句謂語為： (should) + 動詞原形。
You let the phone ring twice before answering, lest you appear overly eager.
讓電話先響兩下再接，以免顯得過於急切。
（陳述句）
At the time, granny followed me everywhere lest I get hurt!
當時，奶奶到哪兒都跟著我，免得我受傷！
（虛擬語氣）
虛擬語氣這些年在英語表達中使用的頻率已經開始下降。原因可能有兩個：其一，像漢語等語言一樣，不用虛擬結構，交流雙方也能很好地理解對方是陳述事實還是假設；其二，它的使用規太複雜。事實上，作為一種語言工具，虛擬語氣是相當低效的，而語言的發展的趨勢，效率總是勝過教條。
儘管如此，虛擬語氣中動詞的變化對本族人還說還說很有魅力的，所以我們還是應該學會使用它。
圖片來自網絡，版權為原作者所有！