如：If the doctor had been available, the child could not have died.

　　There is a real possibility that these animals could be frightened, should there be a sudden loud noise.

　　(2)區分主從句表示的不同時間概念：

　　主從句謂語動詞所指時間不同，這叫做錯綜時間條件句，動詞形式應根據實際情況來調整。如：

　　Had it not been for the timely investment from the general public, our company would not be so thriving as it is.

　　(主句與現在事實相反，從句與過去事實相反)

　　Had Paul received six more votes in the last election, he would be our chairman now.

　　(主句與現在事實相反，從句與過去事實相反)

　　(3)識別事實和假設混合句：

　　Your math instructor would have been happy to give you a make-up examination had you gone and explained that your parents were ill at the time. (句子前半部分為假設情況，而“父母病了”是事實) I would have gone to visit him in the hospital had it been at all possible, but I was fully occupied the whole of last week. (前半部分為假設，后半部分是事實)

　　2.名詞性從句的虛擬形式

　　名詞性從句是指賓語從句、主語從句、表語從句和同位語從句。從句的謂語動詞需用(should+)動詞原形表示虛擬。考生應熟悉：

　　(1)下列動詞做謂語時，that賓語從句中的動詞用虛擬形式：

　　desire, advise, recommend, command, direct, order, ask, demand, request, require, insist, maintain, move, propose, prefer, urge, vote。如：

　　In the past men generally preferred that their wives work in the home. I move that he be discharged for his serious mistake.

　　(2)下列形容詞和分詞做表語或補語時，that主語從句中動詞用虛擬形式：

　　advisable, desirable, insistent, preferable, urgent, appropriate, compulsory, crucial, essential, imperative, important, necessary, obligatory, possible, probable, proper, vital, advised, arranged, commanded, demanded, desired, ordered, proposed, recommended, requested, required, suggested。如： The board deem it urgent that these files should be printed right away. It is essential that all these figures be checked twice.

　　(3)下列名詞接同位語從句或表語從句時，從句中動詞用虛擬形式：

　　insistence, preference, recommendation, suggestion, proposal, motion, desire, requirement, request, order, necessity, importance, regulation, rule, resolution, understanding。如：

　　John Wagner’s most enduring contribution to the study of Afro-American poetry is his insistence that it be analyzed in a religious, as well as worldly, frame of reference.

　　They keep telling us it is of utmost importance that our representative be sent to the conference on schedule.

　　3.含蓄虛擬條件句的謂語動詞形式

　　含蓄虛擬語氣是指假設條件不通過if從句表達，而是暗含在其他結構中。考生應熟悉：

　　(1)連詞but, but that, or, or else;副詞otherwise, unfortunately等表示轉折假設。如：

　　A safety analysis would have identified the target as a potential danger. Unfortunately, it was never done. Victor obviously doesn’t know what’s happened; otherwise he wouldn’t have made such a stupid remark.

　　(2)介詞短語暗含假設條件，常用的有：without, but for, under more favorable conditions等。如：

　　But for the English examination I would have gone to the concert last Sunday.

　　(3)intended/meant/hoped/wished/plann或was/were +不定式完成式或had intended/meant/planned/hoped/wished +不定式一般式暗示虛擬語氣。如：

　　I intended to have called on you, but I was busy at that time.

　　(4)情態動詞完成式暗示虛擬語氣。如：

　　I should have called to make an airline reservation, but I didn’t.

　　4.常用虛擬形式的句型

　　(1)從句中動詞用過去式或過去完成式表示虛擬的句型：

　　would rather would as soon as though suppose… had rather would sooner as if supposing… If only… It is (high) time that…(從句中動詞只用過去式) 如：

　　His wife would rather they didn’t talk about the matter any more.

　　If I were in a movie, then it would be about time that I buried my head in my hands for a cry.

　　(2)If it were not for… (與現在事實相反)、If it had not been for… (與過去事實相反) 相當于but for。如：

　　If it had not been for his help (= but for his help), we would not have succeeded.

　　(3)If only…謂語動詞視情況選用適當的形式。如：

　　If only the committee would approve the regulations and put them into effect as soon as possible.

　　(4)lest/for fear that/in case 從句謂語用(should+)動詞原形。如：

　　The mad man was put in the soft-padded cell lest he injure himself.

　　(5)whether…or…有時謂語用be的原形，引導讓步虛擬從句，這種用法經常采用倒裝結構。如：

　　Church as we use the word refers to all religious institutions, be they Christian, Islamic, Buddhist, Jewish, and so on.