条件和假设

一、真实条件从句

从句提出的条件是能实现的。如果该条件实现，则主句所表示的结果也将实现。例如：

1. If a = b and b = c, then a=c. 如果 a=b, b=c, 那么 a=c.
2. If the temperature of water is below 0℃, it exists in the solid state. 如果水温低于0℃，水就处于固态。
3. If Henry comes here, I shall ask him some questions. 若亨利来这里，我将问他一些问题。

二、假设或非真实主从句

从句所表示的假设与事实相反，或者不大可能实现. 一般可分为下列三种假设情况。这时谓语动词要用虚拟语气，以表示所说的话只是一种假设、愿望、猜测而已。（参阅第144节一、三、四、五和六。）

I. 与现在事实相反的假设：这时从句中谓语用过去一般时（be 用 were），主句谓语是“should 或 would（或其它情态动词 could, might 等）+ 动词原形”

1. If Joe had time, he would help you. 假如乔有时间，他就帮助你了。（事实上他现在没有时间。）
2. If I knew English, I should be able to read these English books. 假如我懂英语，我就能读这些英语书了。（实际上我现在不懂英语。）
3. If there were no gravity, there would be on air around the earth. 若没有重力，地球周围就没有空气。（事实上有重力。）
4. If there were no electronic brains, we could not live a modern happy life. 假如没有电脑，我们就不能过现代化的幸福生活. （假设的从句也可换用 If it were not for electronic brains. 详见本节、三、Ⅲ。）
5. Suppose (= If) you had a sum of money, what would you do with it? 假如你（现在）有一笔款子，你会用来做什么呢？（事实上现在你没有一笔款子。）
6. What would happen if human cloning became a reality? 万一人类无性生殖（指克隆人）成为现实，那将会发生什么呢？

II. 与过去事实相反的假设：这时从句谓语用过去完成时，主句谓语是 “should 或 would（或其它情态动词could，might 等）+ 完成时（后者一律用“have + 过去分词”形式）”。

1. If we had known the basic principles. we should have controlled the process even better. 假如我们知道这些基本原理，我们早就能更好地控制这个过程了。（事实上这些基本原理我们当时还是不清楚的。）
2. If you had come here yesterday, you would have seen him. 假如你昨天来这里，你就见到他了。（事实上你昨天没来这里。）
3. If she had further considered the problem, she might have come to the correct conclusion. 假如她当时能进一步考虑这个问题，她也许（会得出正确的结论。（事实上她当时没作进一步考虑。）

III. 与将来事实可能相反（指不可能实现或可能性较小）的假设：这时从句谓语用“should（或其它情态动词could, might 等）+动词原形”或“were + 带 to 的不定式动词”，主句谓语为“should, would 或其它情态动词过去式 could，might 等 + 动词原形”。

1. If Linda should come to us, she would answer these questions. 假如琳达真能来我们这里的话，她会回答这些问题的。（事实上她来的可能性较小或者不存在。）
2. If Sidney were to do this test, he might do it (in) some other way. 要是锡德尼做这项试验，他可能用另外某种方法做。（实际上不是他将去做这项试验。）

三、假设情况的其它表示法

Ⅰ. 用颠倒主、谓语词序手段表示假设条件从句：假设条件从句中谓语有 were, had, should, could 等时，则可去掉 if，把这些词提到从句最前面，即可构成假设条件从句的另一种句型。例如：

1. Were there no gravity, there would be no air around the earth. 假如没有地球引力，地球周围就没有空气.
2. Were Jenny in Tianjin, she would help us. 假如詹妮在天津的话，她是会帮助我们的。
3. Had Dr. Ouyang time, he would help us (to) solve these problems. 假如欧阳博士有时间，他就会帮我们解决这些问题了。
4. Should they come to us, I should ask them some questions. 假如他们真能来我们这里的话，我将问他们一些问题。
5. Had the checking up of the experimental data not taken so much time, we should have completed our work long ago. 假如检验实验数据不花那么多时间，那么我们早就完成这项工作了。
6. Could he have fore seen it, his iron soul must not have been shaken. 假如他当时预知这件事，他铁一般的意志就决不会动摇了。
7. He might live quite comfortably were it not for such troubles. 若没有这样一些麻烦，他会过得很舒服的。
8. People might have avoided many accidents had ultrasonic methods been known before. 要是人们早就知道超声波方法，就可避免许多事故。
9. Did I see him, I would blame him for it. 如果我现在见到他，我就会为那件事责备他了。
10. Many of the successful men, had they been able to choose for themselves, would have selected other different professions. 许多成功的人，如果当初他们能自由选择职业的话，他们会选择其它不同的职业。
11. Should he call, tell him I am not at home. 万一他来叫我，就告诉他，我不在家。（本句假设是对将来事实可能相反的假设，只是实现可能性极小，但毕竟还有一点，故后面主句也可不用虚拟语气。）
12. Should they come (= If they should come), I shall speak to them about it. 如果他们真的来，我就给他们讲讲这件事。（假设的情况与上句相同，还有一点可能性实现，故后面主句也可用将来时。）

现将上述三种假设情况汇总成下列一表：