IF是一個英語單詞，名詞、連詞，作名詞時意思是“條件，設想;不確定的情況”，作連詞時意思是“(表條件)如果;(表假設)要是，假如;無論何時;雖然，即使;(用于間接疑問)是否;(用于禮貌問詢)是否;當，每次;(與情態(tài)動詞連用)表婉轉客氣;這不是……，要是……多好;或許不;就算;盡管”。那么，if的虛擬語氣用法你了解嗎?

if 條件狀語從句的虛擬語氣：

一.基本結構

1. 與現在事實相反

If + 主語+were/ did/ were doing, 主語+should/ would/ could/ might +動詞原形

強調：必須把上面的結構套用對!

2. 與過去事實相反

If + 主語+ had done, 主語 +should/would/ might/ could + have done.

強調：必須把上面的結構套用對!

3. 與將來事實相反

If + 主語 + were/ did/ were to do/ should do, 主語+ should/ would/ could/ might do

強調：必須把上面的結構套用對!

第三個結構中，從句中的were to do可能性較小，should do的可能性稍大。

二.變化

1. 錯綜時間條件句

所謂錯綜，就是結構和我們的基本結構不一致，出現了從句用與過去事實相反，而主句則為與現在事實相反或與將來事實相反。

那么，我們就分別用各自的結構即可。

If + 主語+had done, 主語 +would/should/ could/ might do

再將強調結構必須正確。

我們對比下面兩個句子：

If you had studied harder then, yoursituation would have been better.

If you had studied harder then, yoursituation would be better now.

注意：在錯綜時間條件句中，務必有明確的表示錯綜的時間狀語，或者通過上下文能看出是時間的不同，否則，就必須按前面的基本結構套用。

2. 含蓄條件句

假設情況并不一定用條件從句來表示，而用其他方式來表示，這就是含蓄條件句了。

用without短語，分詞短語，或獨立主格來表示。

Without going with you last night, I wouldhave been robbed.

=If I hadn’t gone with you last night, Iwould have been robbed.

Having known each other before, we wouldn’thave fought last night.

= If we had known each other before, wewouldn’t have fought last night.

用相當于if的其他連詞表示。

But that...要不是

Otherwise否則

He was ill, otherwise, he would have cometo meet you.

通過上下文來判斷。

I would have given you more money, but Ihad little money last month.

三.虛擬條件句中的倒裝

滿足如下三個條件方可倒裝：

必須是非真實條件句

從句中的謂語動詞有were, had,should時

倒裝后，省略引導詞if,

Had I a lot of money, I would buy a bighouse.

=If I had a lot of money, I would buy a bighouse.

if引導條件狀語從句的用法：

某事很可能發(fā)生,條件是可能存在的,主句中某種情況發(fā)生的概率也是很高的.如：

If you ask him,he will help you.如果你請他幫忙,他會幫你的.

If you fail in the exam,you will let him down.如果你考試不及格,你會讓他失望的.

If you have finished the homework,you can go home.如果你作業(yè)做完了就可以回家了.

另外,if從句還表示不可實現的條件或根本不可能存在的條件,也就是一種虛擬的條件或假設.從句多用一般過去時或過去完成時,表示對現在或過去的一種假設.如：

If I were you,I would invite him to the party.如果我是你,我會邀請他參加聚會.

I would have arrived much earlier if I had not been caught in the traffic.要不是交通堵塞,我本會來得早一些.

另外你還要注意if 條件句的時態(tài)搭配

1.if從句用一般現在時,主句用一般將來時

If he runs he’ll get there in time.如果他用跑的,他就會及時趕到那兒.

The cat will scratch you if you pull her tail.如果你拉貓的尾巴,它就會抓你.

2.if從句用一般現在時,主句用may/might/can

If the fog gets thicker the plane may/might be diverted.如果霧在大一些,飛機可能就會改在別的機場降落.

If it stops snowing we can go out.如果雪停了,我們就可以出去.

3.if從句用一般現在時,主句用must/should

If you want to lose weight you must/should eat less bread.如果你想減肥,你必須少吃面包.

4.if從句用一般現在時,主句用一般現在時

If you heat ice it turns to water.(也可用will turn)如果把冰加熱,它就會化成水.

5.if從句用現在進行時,主句用一般將來時

If you are looking for Peter you’ll find him upstairs.如果你是在找彼得,上樓就會找到他.

6.if從句用現在完成時,主句用一般將來時

If you have finished dinner I’ll ask the waiter for the bill.如果你吃完了,我就叫服務生來算賬

虛擬語氣的用法：

虛擬語氣是一種特殊的動詞形式,用來表示說話人所說的話并不是事實,而是一種假設、愿望、懷疑或推測.判斷是真實條件句還是非真實條件句.只有在非真實條件句中才使用虛擬語氣.通過句子意思,看假設的條件是否能夠實現,能夠實現是真實條件句,不能使用虛擬語氣;假設的條件不能實現則是非真實條件句,要用虛擬語氣.判斷這個假設是與哪個事實相反.

通常有三種情況：①與過去事實相反.②與現在事實相反.③與將來事實可能相反.

I: 較基本的虛擬語氣句型:

1. 虛擬現在時表示與現在事實相反的假設,其if 從句的謂語形式用動詞的過去式(be 一般用were),主句用would/ should/ could/ might +動詞原形.If places were alike, there would be little need for geographers.如果各個地方都一樣,就不需要地理學家了.

2. 虛擬過去時是表示與過去事實相反的假設,if 從句的謂語形式用過去完成時即had+過去分詞,主句用would / should / could / might + have +過去分詞.If he had known your address yesterday,he would have telephoned you.

3. 虛擬將來時是表示對將來實現的可能性很小的或不確定的假設.If從句的謂語形式用一般過去式或用were to / should +動詞原形,主句用would / should/could/ might +動詞原形.If he were to leave today, he would get there by Friday

4. 省略if 采用倒裝語序的條件句.有時可以把含有助動詞、情態(tài)動詞、be或have的虛擬條件句中的連詞if省去,而將had , should, were 等詞提到主語之前,即用倒裝結構.If he had worked harder, he would have got through the exams.==>Had he worked harder, he would have got through the exams.If he were to leave today, he would get there by Friday.==>Were he to leave today, he would get there by Friday.If I were in your place, I wouldn’t do that.==>Were I in your place, I wouldn’t do that.

5. 有時虛擬條件句并沒用if 從句表示出來,而是用介詞短語(otherwise, or, without, but for)、上下文或其它方式來表示.如：We didn't know his telephone number; otherwise we would have telephoned him. Without you help, I wouldn’t have achieved so much. But for (“要不是.”的意思)your help, I would not have succeeded.

II: 虛擬語氣在從句中用should的情況:

1. 在表示建議,要求,命令,想法的動詞后的從句里, 用should + 動詞原形, should 可以省略. 如 advise, agree, command, decide, demand, determine, grant, indicate, insist, order, prefer, propose, request, require, stipulate, suggest, urge, vote.常考到的是: suggest, advise, demand, require, propose, insist, order, request.

His doctor suggested that he (should) take short leave of absence.

The author proposed that TV (should) be turned off at least one hour every day.

2. 在表示建議,要求,命令,想法的名詞后的從句里, 用should + 動詞原形, should可以省略. 如 advice, decision,agreement, command, decree, demand, determination, indication, insistence, order, preference, proposal, request, requirement, stipulation etc.

It was Bill’s suggestion that everyone (should) have a map.

His suggestion was that everyone (should) have a map.

He gave us a suggestion that everyone (should) have a map.

3. 在It is/was +形容詞后的that 從句中用should的結構, should 可以省略. 這類形容詞常見的有: advisable, anxious, compulsory, crucial, desirable, eager, essential, fitting, imperative(必要), impossible, improper, important, natural, necessary, obligatory, possible, preferable, probable, recommended, urgent, vital etc.

It‘s natural that she (should) do so.

It is essential that we (should) tell her the news.

4. 在 lest 和 for fear that(以免),in case(以防)從句中用should, should 可以省略.

She walked quietly lest she (should) wake up her roommates.

Have your gun ready in case we should need it.

III: wish 后的 that 從句中:

1. 表示現在或將來的愿望,從句中過去式.

I wish I knew his address.

I wish I were young.

2. 表示過去沒有實現或不可能實現的愿望, 從句用過去完成式或would, could, might + have + 過去分詞.

I wish you had written to him.

I wish I could have slept longer this morning, but I had to get up and come to class.

3. 如果將wish改成wished, 其后that 從句中動詞的形式不變.

4. 如果that 從句中用would , 一般表示 對現狀不滿或希望未來有所改變或請求

I wish he would answer my letter.

I wish prices would come down.

I wish you would help me.

I wish you would stop asking silly questions.

IV: 在 It’s about/ high/ first(second, third etc)time 后的that 從句中,用過去式.表示“該是做什么的時候了”

It is about time you were in bed.

It is high time we left.

It is the first time I came here.

V: 在would rather, would sooner, would just as soon 后的that從句中,用過去式或過去完成式,表示“寧愿做什么”

I would rather he came tomorrow than today.

John would rather that she had not gone to the party yesterday evening.

VI: 在if only(“如果.就好了”的意思) 感嘆句中,謂語動詞與wish賓語從句的虛擬形式相同.

If only he didn’t drive so fast!(現在)

If only she had asked someone’s advice.(過去)

If only the rain would stop.(將來)

VII: 在as if / as though 從句中,表示和現在事實相反或對現在情況有所懷疑,用過去式;表示過去想象中的動作或情況, 用過去完成式.

He speaks as if he were on the spot.

She spoke to me as if I were deaf.

This device operated as though it had been repaired.

注: 1. 在 as if / as though 句中, 如果有可能成為事實, 用陳述語氣.

He looks as if he is going to be ill.

2. 在 insist 后的從句中, 如果是堅持自己, 用陳述語氣, 堅持別人做什么事情, 用虛擬語氣.

She insists that she is right.

She insisted that I should finish the work at once.

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