虚拟语气中的含蓄条件句常见用法
先看一道高考题：
____ more attention，the trees could have grown better．
A．Given B．To give C．Giving D．Having given
（答案是A）
句中"．．．the trees could have grown better"是一个表达虚拟语气的句子。
题中要求选择的是如何表达假设条件的形式。
如果我们把这个句子用另一种形式"If they had been given more attentio n ，the trees could have grown better．"来表示，就不难理解了。
很多情况下，在表示假设的虚拟语气中，有时假设的条件并不以条件从句的形式表达，而是通过上下文或句中内在的逻辑关系，或用其它形式表达出来，我们称之为"含蓄条件句"。
含蓄条件句主要通过名词、连接词、不定式、分词、动名词短语、介词短语及分句来暗示。
这些表达形式在句中实际起着虚拟条件从句的作用。
现分述如下：
一、介词短语
常用来表示虚拟条件的介词有with，without，in，under，but for等等，它们表示的虚拟条件都可以被if从句替换。
（1）without，with
without表示否定的条件，意为if．．．not；with与without意义相反，表示肯定的条件。
如：Without air，there would be no living things．（without air ＝if there were no air）没有空气，便没有生物。
With her help（＝If I had her help），I would do the experiment well．假如有她帮助，我就可以把这个实验做好。
（2）under
Under the leadership of a less experienced person，the experiment would have failed．（u nder．．．＝If there had been the leadership of a less experienced person）假设在一个缺少经验的人领导下进行实验，实验准会失败。
（3）in
I would have lost my head in that position．（in that position ＝if I had been in that po sition）我处在那种情况下，是会丧失理智的。
（4）but for
But for their help（＝If it were not for their help），we could not get over the difficultie s．要不是他们的帮助，我们是克服不了这些困难的。
二、连接词or ，otherwise，but，but that等等
I'm really busy，otherwise（＝if I weren't so busy），I would certainly go with you．我确实太忙了。
不然的话，我一定和你一块去。
But that I saw it（＝If I had not seen it），I could not have believed it．要不是亲眼看见的话，我是不会相信的。
三、分词短语
United（＝If they had been united），they wouldn't have been defeated．他们要是团结起来，就不会被打败。
Seen from a high mountain（＝If it were seen from a high mountain），the field in whi ch wheat is growing would look like a great green sea．倘若从高山上看，麦田就像一片碧绿的大海。
四、动词不定式短语
It would be only partly right to answer in this way．（＝if we answered in this way）如果这样回答，只对一部分。
To hear him speak English（＝If one were to hear him speak English），one would think him an Englishman．要是听他讲起英语来，人们会认为他是一个英国人。
五、形容词或其比较级的形式
A less difficult problem would have already been solved．（＝If the problem had been l ess difficult，it would have already been solved．）要是问题不那么难的话，它早就被解决了。
An honest man would not do such a thing．（＝If he were an honest man，he would no t do such a thing．）诚实的人是不会做这种事的。
六、名词
An honest man would not say this．（＝If the man were honest，he wouldn't say this．）一个诚实的人，就不会说这种话。
Who but a fool（＝If one were a fool，one）would believe his words．除了傻瓜，谁会相信他的话！
七、"名词＋and"的结构
One step further and you would be dead．（＝If you took one step further，you would be dead．）再往前走一步，你就会死。
八、独立主格结构或由with引导的复合结构
All things considered（＝If all things were considered），the price would be reasonable．如果全面考虑起来，价格可能是合理的。
九、定语从句
A nation which stopped working would be dead in a fortnight．（＝If a nation stopped working，it would be dead in a fortnight．）如果一个国家停止工作，不出半个月就会灭亡。