They talked as if they had been friends for years. She looks as if she would cry.

(3)用来表示要求、建议、命令、提议等的主语从句、宾语从句和同位语从句中的虚拟形式：should + 动词原形，should可省略

1) 用在表示建议、命令、要求、忠告等动词后，动词形式为(should) +动词原形. 常见动词：一个坚持(insist)，两个命令(order, command)，三个建议(advise, suggest, propose)，四个要求(demand , require, request, desire)。

注意：在suggest后的从句中 A.如果表示建议，用虚拟语气

B.如果表示表明、暗示，用陈述语气

The doctor suggested that I should take the medicine three times a day.

The doctor suggested that my grandmother had caught a bad cold.

在insist后的从句中 A.如果坚持做什么事情，用虚拟语气

B.如果坚持表明，坚持说/解释，用陈述语气

My sister insisted that she should organize the trip.

He insisted that he was right.

2)用在“It is (was)+形容词(或过去分词)/ 名词 + that……”结构中：

常用形容词有：natural, necessary ,important, strange, possible, surprising, amazing，desirable合意的，advisable明智的，crucial关键的，insistent坚持的，essential紧要的，基本的等

常用过去分词有: required ,demanded, suggested 建议, recommended 推荐, commanded命令的，desired想要，ordered命令，proposed提议，requested要求的等

It is essential that he should be prepared for this.

It is requested that Mr. Li give a performance at the party.

3)用于由表示建议、要求、命令请示等含义的名词引导的表语从句和同位语从句中，这类名词常见的有：advice, demand , desire, suggestion, proposal , request , order, requirement, recommendation ,motion提议，resolution决心，decision，idea，pity，preference，shame， surprise，wish，wonder等。

(4) 用在would rather/sooner, had rather, would(just) as soon引导的宾语从句中,表示‘‘宁愿做什么”或对过去做的事的懊悔。

1)表示与现在或将来事实相反用过去时

2)表示与过去事实相反用had done

I’d rather you posted the letter right away.

I would just as soon you had returned the book yesterday.

注意①若某人愿自己做某事，would rather后用动词原形。 I would rather stay at home today.

②would rather...than...中用动词原形。 I would rather stay at home than go out today.

(5) It is (about/high)time + 过去式/should+动词原形 (should 不能省)

It is high time that we put an end to discussion.

(6) 在for fear that …/ in case that …/ lest以免，万一等引导的状语从句,动词用should+动原形，should 可以省略。

She put a blanket over the baby for fear that he should catch cold.

3. 含蓄虚拟条件句

含蓄虚拟条件句是指句中没有明显的虚拟条件句，而是利用其他手段来暗示存在虚拟条件，这类隐含式虚拟条件句往往可以转化成if引导的条件句。常用来表示含蓄虚拟条件的手段有：

(1)介词或介词短语，如：but for, but that, without等

But for the leadership of the Party, we could not be living happily today.

= If there were not the leadership of the party, we could not be living happily today.

Without music, the world would be a dull place.

(2)通过上下文及内在含义，句中往往有but, otherwise等类似转折词。

He worked very hard, otherwise he would have failed.

(3)分词短语

Born ten days earlier, the boy could have seen his father.

(4)情态动词完成时暗示虚拟语气。

should/ could / might/ ought to + have done 表示“过去本应该可以做而实际上却没做”

shouldn’t have done表示“过去不应该做而实际上做了”

needn’t have done 表示“过去没必要做而实际上做了”