【问】 If increased production of electricity by nuclear reactors... enables it to remain economical...we may soon reach the crossover point for hydrogen... The second attractive use ofhydrogen would be to produce electricity by fuel-cell conversion. (上海交大《英语》(三)，1979年版，P.312)
 [原译]……如果用核反应堆……来增加发电量使电保持廉价的话，我们很快就可以转而生产氢了……氢的第二种引人注目的用途也许是通过燃料电池换能法来发电。(教参)
 这里并没有非真实条件从句，为什么句子动词用虚拟语气would be?另外，would在这里是表达“也许”的意思吗?
 【答】 要想探讨这里的虚拟语气问题，必须弄清英语动词虚拟语气在非条件句中的用法和意义。我们在教学中发现，许多学生往往不懂得这类虚拟语气的功用，因而常常产生一些误解。例如，有不少学生不能理解下列句子的谓语动词使用虚拟语气的原因，因而不能确切理解句子的真正含义：
 1. The crew's deaths would have been less likely in peacetime because the crew could haveradioed their position. (Reading Laboratory，Ⅲb，Book 6，P. 15)
 如果是在和平时期的话，机组人员是不大可能死亡的，因为他们可以用无线电报告他们的位置。
 2. A list of all the ways in which diesel power is used would take pages，and engineers keepfinding new uses for these engines.
 要是把柴油机的所有用途都一一列举出来的话，一定会占好几页的篇幅。
 这里应当首先指出，除了if，unless，in case，as long as，suppose (supposing)，on conditionthat，provided that，even though能引出非真实的条件之外，英语里还有许许多多其它语言手段能够表达非真实的情况，即所谓“假想”的情况。
 A.介词短语
 1. Without steel，there would be no industry.
 没有钢就没有工业。(事实是：有钢)
 2. But for friction，people could neither walk，nor even stand up.
 如果没有摩擦，人们就不能走路，甚至连站也站不起来。(事实是：有摩擦)
 3. She would leave her husband except for the children.
 如果没有孩子她就会离开她丈夫。(事实是：她有孩子)
 4. Because of his arguments he was convinced that friction provided the forces，which stopbodies in horizontal motion，and that in the absence of all forces the bodies would con-tinue to move forever.
 由于这个论点他认为是摩擦提供了力，使水平运动的物体停止下来，假如没有任何力的话，物体就会永远继续运动下去。(事实是：不可能没有摩擦力)
 5. They could have achieved more under better conditions.
 如果条件较好的话他们本来会取得更大的成就的。(事实是：条件不好)
 6. Man’s life would be impossible in a world free from friction.
 在没有摩擦的世界里，人是无法生活的。(事实是：没有摩擦的世界是不存在的)
 7. But for John they would all have died. (CGEL)
 如果没有约翰，他们大家都死了。
 8. Except for John，they would all have died. (CGEL)
 如果没有约翰，他们大家都死了。
 上面各例句中的介词短语，实际上起着与非真实条件状语从句相同的作用，因而都可以转换为非真实条件状语从句。例如，例2可以转换为：If there were no friction，people couldneither walk，nor even stand up.但是，介词短语比非真实条件状语从句简洁、紧凑，因而在书面英语里用得颇为广泛。明确这一点，便可以很容易纠正某些误解。例如，有一本英语语法著作(《科技英语结构分析与翻译》)是这样解释下列句子里的would的用法的。
 “The weight of the body，the earth pull on it，depends on the location of the body on theearth and of course would be quite different on another planet.
 物体的重量，即地球对它的引力，与该物体在地球上的位置有关，当然，该物体的重量在另一星球上应当是完全不同的。
 (would...是虚拟语气，有‘应当’，‘理应’之意，语气委婉。)”
 读者很难理解，在这个表达科学内容的句子里，语气为什么要“委婉”。读者也很难理解，为什么would偏偏在这个句子里具有“应当”、“理应”之意。是不是这里的would be与一般的虚拟语气不同?其实，只要我们抓住介词短语可以表示假想情况(虚拟的条件)这个关键，就能很容易解释这个句子里的虚拟语气的用法。原文中的介词短语on another planet表示的是一种假想的情况，因为该物体(the body)并没有真的放到另一个行星上，因此，on anotherplanet相当于if it were on another planet。
 还应指出的是，在英译汉时，往往需要把介词短语所表示的假想意味给表达出来，否则可能有歧义。例如：
 It should be noted that a rocket does not depend on the atmosphere for its propulsion. but-would actually perform better in the absence of an atmosphere because of lessened air resistance.
 [原译]应该注意，火箭的推力与大气无关，在没有大气的情况下，由于空气阻力减小，火箭实际上会运行得更好。(《科技英语结构分析和翻译》，P.489)
 原文中的介词短语in the absence of an atmosphere表示虚拟的条件，即表示与客观现实相反的假想情况，译文译作“在没有大气的情况下”，未能把这种假想情况表达出来。因为人们有可能把“在没有大气的情况下”理解为“在火箭脱离大气层以后的情况下”。
 [试译]应该指出，火箭推力的产生是与大气无关的，但是，假如没有大气的话，由于空气阻力减小，火箭实际上会运行得更好。
 请读者自己分析为什么下列四个句子都用了虚拟语气。
 Without air，there would be no wind or clouds. There would be no weather，as we knowit. The night time would be very cold and the days very hot. We would be forced to seekshelter from the sun，as there would be no atmosphere to protect us from the sun's deadlyrays.
 B.动词不定式短语
 1 )充当主语的动词不定式短语
 充当主语的动词不定式常常含有假想的意味，也就是说，它所表达的内容并不是现实的东西，而是想象中的东西或是彼时彼地不能实现的情况。在这种情况下，句子的谓语动词常常用虚拟语气。例如：
 1. Aware of the fact that it would be impossible to wait thousands or millions of years to re-ceive an answer from a distant planet，scientists engaged in Project Ozma are concentrating their attention on stars which are relatively close. (New Concept English，Book 3，P. 134)
 参与奥斯玛计划的科学家们知道，人们不可能等待几千年或者几百万年去接收来自遥远星球的回讯，所以便把注意力集中到比较近的星球上。(动词不定式短语towait... planet所表达的情况是根本不可能实现的)
 2. It would be wrong not to take a thorough study and investigation before the planning anddesigning.
 在规划和设计之前不作充分的调查研究，是错误的。(动词不定式短not to take...designing所表达的事情，是想象的)
 3. In industry，for example，television cameras are used to watch processes in places，where it would be difficult or even dangerous for a human to be present.
 例如，在工业上，电视摄象机可以在人难以到达或者人在那里有危险的地方来监视生产过程。(作者把for a human to be present所表达的情况仅仅看作是假想的，而不把它看作是实际上可以实现的)
 4. The molecules of water are incredibly minute. It would take about 125 million of themto form a row an inch long.
 水分子小得难以置信。把水分子排成一英寸长就需要大约一亿二千五百万个分子。
 (动词不定式短语所表达的事情是假想的，不是真的要去做的)
 5. It would be a mistake to think that a quick elementary study of physics can explain thepath of a jet plane in the sky，or the intricate rhythms of a diesel engine.
 如果认为很快地学点物理学的初步知识，就能阐明喷气式飞机在天空中的飞行轨迹或柴油机复杂的运动，那就错了。(作者把to think…仅仅看作是假想的情况)
 6. It would be unwise for you to marry him. (=if you were to marry him.)
 要是你跟他结婚，那将是不明智的。
 It would be foolish，not to say mad，to sell your car. (LDCE)
 [原译]把你的车子卖掉是很不明智的，甚至可以说疯了。(《朗文现代英汉双解词典)，现代出版社，P.954)
 从译文看，似乎“车子已经卖掉”，但原文的虚拟语气告诉我们：to sell your car只是一种假设情况。
 [试译]你要是把你的汽车卖掉，即使不是发疯，也是很愚蠢的。
 2)充当条件状语的动词不定式短语
 我国出版的许多英语教科书和英语语法著作在讲动词不定式充当状语时，一般只提到动词不定式能够充当目的状语，结果状语和原因状语，大都没有提到能够充当条件状语。因此，很多学生乃至不少英语教师不能很好地理解或阐述动词不定式表达虚拟条件的句子
 1. You would do well to write more distinctly.
 你要是写得再清楚一些就好了。(动词不定式短语to write more distinctly是条件状语，而且这个条件是和现实相反的)
 2. When I was a teenager，I felt that I was just young and uncertain—that I was a new boyin a huge school，and I would have been very pleased to be regarded as something so in-teresting as a problem.
 我十几岁时，总感到自己很小，茫然无知——就好象一所大学校里的新生，假如那时我被看作是一个难以对付的人物而惹人注目的话，那末，我是会感到非常得意的。
 (to be regarded…表达的是一个与过去事实相反的假想条件)
 3. To be launched in a rocket you would have to lie down.
 如果你乘坐火箭被发射的话，你就得躺下。(作者认为在他讲话的时候“你乘坐火箭”这件事，并非是现实的，而仅仅是假想的，故动词不定式短语to be launched in a rock-et起着与非真实条件状语从句相同的作用)
 4. In the Union of South Africa it is very difficult for a native to get a skilled job. To quali-fy for such a position，the native would first have to receive specialized training，andthis is prohibited.
 在南非，黑人很难找到技术性工作，黑人要想有资格获得这样的工作岗位，就必须首先受专门的训练，而这是禁止的。
 C.名词或名词短语
 1. The list of discoveries by accident could fill a long book; and remember，most of themhappened when somebody asked himself： “why?”
 要是把偶然的发现都一一写出来的话，恐怕要写上厚厚的一大本书。并且要记住，大部分偶然的发现都是在有人向自己问一个“为什么”的时候产生出来的。(事实上作者或者其他人并没有真的把所有的偶然发现都一一写出来，也就是说，在作者写这句话的彼时彼地并不存在the list of discoveries by accident。因此，the list仅仅是作者假想的东西)
 2. Physicists have a strong wish for a simple but sound theory that would explain the ulti-mate physical make-up of matter.
 物理学家非常希望有一种简单而可靠的理论来解释物质的最终物理结构。(a simplebut sound theory仅仅是人们希望其存在但实际上当时并不存在的东西，故它的定语从句中的谓语动词用虚拟语气)
 3. For this reason，intelligent creatures in any part of the universe would be able to under-stand a simple arithmetical sequence. They would be able to reply to our signals usingsimilar methods. (NCE 3，P.134)
 因此，宇宙中其它任何地方的有智慧的动物都能理解简单的算术序列。他们能够使用类似的方法来回答我们的讯号。(迄今尚未证明其它星球存在有智慧的生命，因此intelligent creatures in any(other) part of the universe仅仅是假定其存在的东西)
 4. For several years the author has felt in his teaching the need for a book which would pro-vide a thorough and rigorous introduction to college physics for students who intend topursue scientific careers.
 作者多年来在教学中感到需要有这样一本书，这本书会给打算从事科学事业的学生提供系统而严谨的大学物理学的入门知识。
 5. There is no necessity to destroy the basic word stock when it can be effectively usedthrough the course of several historical periods，not to speak of the fact that，it beingimpossible to create a new basic word stock in a short time，the destruction of the basicword stock accumulated in the course of centuries would result in paralysis of the lan-guage，in the complete disruption of intercourse between people.
 既然基本词汇能在许多历史时期中有效地利用，那就没有任何必要去消灭它，况且把千百年积累起来的基本词汇消灭掉了，又不可能在很短期间内创造新的基本词汇，那就会使语言瘫痪，就会把人们相互交际的事情完全弄糟。(《马克思主义和语言学问题》，人民出版社)(the destruction...只是假想的事情而并非是客观上的现实)
 6. Imagine the effect on a reasonable advanced technological society，one that still does notpossess the bomb，of making it aware of the possibility，of supplying sufficient details toenable the thing to be constructed. Twenty or thirty pages of in formation handed to anyof the major world powers around the year 1925 would have been sufficient to changethe course of world history.
 如果让一个科学技术相当先进但是尚未拥有原子弹的国家知道制造原子弹的可能性，并向它提供制造原子弹的充分的细节，请设想一下这会产生什么样的作用吧!如果在1925年前后有人向世界上任何一个大国提交了二三十页的情报，就足以改变世界历史的进程。(Twentyor thirtypages ofinformation...是与历史事实相反的假想情况)
 7. The ideal machine which would have an efficiency of 100% should not have any friction.
 效率为100%的理想机器必须没有摩擦。(这种理想的机器在现实里是不存在的，故在其定语从句里谓语动词用虚拟语气)
 8. ... he was prepared to prove his theory that two different weights，dropped simultane-ously from the same height，would fall to ground at the same time. (College English，Book 3，P. 54)
 他准备证明他的理论，从同一高度同时落下的两个重量不同的物体会在同一时间到达地面。(在伽利略进行此项实验之前，人们尚不承认他的理论符合事实)
 D.代词
 He suggested that a double railway tunnel should be built. This would solve the problem ofventilation，for if a train entered this tunnel. it would draw in fresh air behind it. (NCE 2，L. 64)
 他建议建造一个双轨道隧道，这样就能解决通风问题，因为火车开进隧道，就会带进来新鲜空气。
 E.形容词
 1. A true friend would have acted differently.
 如果他是真正的朋友，那他的做法就不会是这个样子了。
 2. A close observer would have noticed it.
 要是细心的观察者，早就注意到了。
 F.副词和副词短语
 1.Living things，as we know them，must have just right amount of oxygen. Too much，and they would burn. Too little，and they would die.
 正如我们大家所知道的那样，生物必须有适量的氧气，氧太多生物会烧死，氧太少生物会窒息死。(too much和too little都是在说话的彼时假想的情况)
 2. Alone Roosevelt could have accomplished little.
 罗斯福要是光杆司令，他也不会有多大作为的。(alone是与历史事实相反的假想情况)
 G.分词短语
 1. A man trying to see a single atom would be somewhat like a man trying to see a singledrop of water in the ocean while flying many miles above it.
 一个人试图看到单个的原子，有点像一个人翱翔于茫茫大海之上许多英里的高空而试图看到大海中的一滴水一样。(tryingto see a single atom完全是一种假想的情况)
 2. The same thing，happening in wartime，would amount to disaster.
 同样的事，要是发生在战争年代，就会酿成大祸。
 H.并列连词 or.otherwise. else，or else
 1. When you stand on the ground you are pushing down on it with a force equal to yourweight，and at the same time the ground must be pushing up on you with exactly thesameforce. Or you would fall all the way down to the center of the earth.
 当你站在地面上时，你就是在向下对地面施加一个等于你体重的力，与此同时，地面必然向上对你施加一个大小完全相等的力，否则你就会一直掉到地球中心。(or提出的是一个与客观事实完全相反的假想情况)
 2. The thin-film cells must demonstrate an efficiency of at least 10 percent，or else the non-cell costs alone would make them too expensive to compete with coal.
 这种薄膜电池必须至少有百分之十的效率，否则仅非电池部分的成本就会使薄膜电池的价格变得非常昂贵，以至不可能与煤相匹敌。
 3. Gas molecules move in all directions，otherwise the color could not spread throughout thejar.
 气体分子是到处运动的，不然的话，颜色就不可能扩散及整个瓶子。
 4. The atmosphere acts as a screen to shield us from certain harmful rays that would other-wise beat down on the earth and make life impossible.
 大气层象屏障一样，保护我们不受某些有害射线的照射，否则，这些有害的射线就会照射到地球上，使生物无法生存。
 I.从属连词only，but，but that，except(that)
 含有虚拟语气的分句描述的是与事实相反的情景，而含有连词but或only等的分句描述的则是事实，这一事实阻止了虚拟语气句所描述的情况成为现实：
 1. Franklin would gladly have spent the rest of his days in quiet study and research. But hewas a very important person now，and the country needed him for public service.
 富兰克林本来是会在安静的学习和研究中愉快地度过他的后半生的，但是他当时是一个非常重要的人物，国家需要他担任公职。
 2. We would have avoided making such a mistake，but we didn’t follow his sound advice.
 我们本来可以不犯这样的错误，只是我们没有接受他的忠告。
 3. He said he would have done，only you treated him so offhand.
 他说他本来会做的，只是你待他太不客气了。
 4. I would've asked you，only my mother told me not to. (CGEL)
 我本来要问你的，只是我妈妈叫我不要问你(要不是我妈妈叫我不要问你的话，我本来会去问你的)。
 5. He would have left but that he was afraid.
 要是他那时不害怕的话，他本来会离开的。
 6. He would have helped us but that he was short of money at the time. (ALD)
 要不是他当时没有钱，他就帮助我们了。
 7. I would go except it’s too late. (NELD)
 要不是太晚的话，我会去的。
 J.定语从句
 1. A nation which stopped working would be dead in a fortnight.
 一个国家要是停止工作，不出两周就会灭亡。
 2. Anybody who should do that would be laugthed at.
 谁要是那样做，谁就会被人耻笑。
 3. Any person who had behaved in that way would have been dismissed. (CGEL)
 如果有谁这样干了的话，早就被开除了。
 K.上下文
 1. For example，suppose that air should leak into an atomic reactor and then leak out again. The air would become radioactive and dangerous for anyone to breathe.
 举例来说，假定空气渗进了原子反应堆然后又渗了出来，那末这种空气就变成具有放射性的了，任何人吸入它都是很危险的。(suppose句提出一个设想的情况，后面的句子是以这个设想的情况为条件而进行叙述的，故其谓语动词用虚拟语气)
 2. In one kind of transistor，three wires are fastened to the crystal of germanium. Let usimaginethat we are using this transistor to amplify a telephone signal. The hookupwould be about as shown on the next page.
 在一种晶体管中有三根金属丝固定在锗晶上。让我们设想我们正在使用这种晶体管来放大电话信号。其试验线路跟下页所表示的大致相同。(imagine句提出一个与彼时彼地的客观事实相反的假想情况，也就是说，事实上在作者写这句话的时候，作者及其读者并没有真的正在使用这种晶体管来放大电话信号。后一个句子以这样的假想情况为其背景的，故谓语动词用虚拟语气)
 3. Let's try a comparison. Reduce the sun to the size of a golf ball. The earth would thenbe the size of a grain of sand.
 我们来打个比方，假如把太阳缩小到高尔夫球那么大，那么地球的大小就会像一粒砂子。
 现在我们已拥有了足够的知识来回过头来探讨你所问的句子的汉译问题了。那里的原文之所以使用虚拟语气，是因为文章的前面提出一种可能性：将来如能使用核反应堆生产廉价的电，我们就可以用电解法制造大量氢。由于这是一种尚未成为现实的可能情况，所以原作者在讨论氢大量生产后的用途时，多次使用虚拟语气，例如，It could supply heat for homesand for industry...the hydrogen could be produced with cheaper offpeak power and burned toproduce power to supply peak demands.就本例而论，充当主语的名词短语the second attrac-tive use of hydrogen，因氢尚未大量生产出来，使用氢来发电仍是一个尚未成为现实的想象情况。因此，它的谓语动词照例应当用虚拟语气，这种用法的would是不能译为“也许”的。因为原作者并不是对“氢的第二种引人注目的用途”究竟是不是“通过燃料电池换能法来发电”抱有怀疑的态度或者没有把握，恰恰相反，原作者对此是毫不怀疑而是有确切把握的。因此，原译文中的“也许”两字应删掉。