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 attention ,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.(under...＝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 position)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 difficulties.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 which 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 less 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 not 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。