作者：陈光明

if既可引导一个真实条件状语从句又可引导一个非真实条件状语从句，二者主要在以下三个方面有所不同：


一、 意义及谓语动词形式不同


如果该条件纯属假设，谓语动词用虚拟语气；有可能实现的条件则使用动词通常的陈述形式：


1．非真实条件状语从句的谓语动词用动词过去式来表示与现在事实相反或难以实现的愿望，主句则用"would+动词原形"；而真实条件状语从句的谓语动词用动词的一般现在时表示一般或未来情形下的条件，主句用一般现在时或将来时。试比较：


If my husband were a woman, he would understand my feelings a bit better. 如果我丈夫是个女人的话，他就会更好一点儿理解我的感情了。(丈夫不可能是女人)


Oil floats if you pour it on water. 如果你把油倒在水上，它就漂浮在上面。（能实现的事实）


2． 非真实条件状语从句的谓语动词用"had+过去分词"表示与过去的事实相反，主句用"would have done"；真实条件状语从句用一般过去时指过去的事实或用于间接引语中（可表示过去将来的情况），主句用一般过去时或过去将来时。试比较：


If I had been a teacher at that time, I would have argued with him. 当时我要是老师的话，就和他争辩了。（事实上当时不是老师）


If Mike was a liar, it was for my sake. 如果麦克曾经是个撒谎的人，那也是为了我。（当时麦克极有可能是撒了谎）


He said he would go there if it was fine the next day. 他说，如果第二天天气好他就去那里。


二、 从句所接的情态动词的含义不同


1． if I (we, you, he...) +" should + 动词原形"或" were to +动词原形"或动词过去式，表示对将来情况的虚拟假设，即某事不会或不太可能发生，具有较大的试探性。如：


If you should run into Peter, please tell him he owes me a letter. 假如你碰见皮特，告诉他他该给我写信了。


What would you do if war were to break out? 如果战争爆发，你会怎么办？


2． if引导的真实条件状语从句一般很少使用将来时态，但有时也可用情态动词will或would来表示"愿望"（如果......愿意......），甚至可用这种方式提出请求。如：


It'll make me very happy if you'll accept it. 如果你愿意收下它我会很高兴的。


If you will wait a moment, I'll go and tell the manager that you are here. 如果你愿意等一会儿，我就会去告诉经理你来了。


If you will hold my bag, I'll open the door (i.e. please hold my bag). 如果你愿意替我拿包，我就开门（即，请替我拿包）。


三、 与其他句型的转换不同


1．非真实条件状语从句中的if可省略，从句可以用倒装语序；而真实条件状语从句一般不可以。如：


If he were my son, I would beat him. 如果他是我的儿子我就打他。→Were he my son, I would beat him. (√)


If you touch me again, I'll call the police. 你再碰我，我就叫警察。→Do you touch me again, I'll call the police. (×)


2. 真实条件状语从句可以转换为"祈使句+and +陈述句"，而非真实条件状语从句则不可以。如：


If you touch me again, I'll call the police.→(You) Touch me again and I'll call the police. (√)


3. 真实条件状语从句的否定式可与unless ...转换，而非真实条件状语从句的否定式却不能。如：


If it doesn't rain tomorrow, I'll plant the flowers. 如果明天不下雨，我就把花栽上。→I'll plant the flowers unless it rains tomorrow. (√)


If it hadn't been for you, I would have failed in the exam. 如果不是你的话我就不会通过考试了。→Unless it had been for you, I would have failed in the exam. (×)