在英文中，有两种定语从句：限定性定语从句与非限定性定语从句。这两种定语从句在其功能和形式方面都有明显的区别：
限定性定语从句限定性定语从句与主句的关系很紧奏，对其先行词起限定、修饰的作用。如果将其去掉，会影响句子意思的完整性；有时甚至于引起费解、误解。例如：
Rainforests are being cleared for valuable timber and other resources to speed up the economic growth of the nationsin which they are located .
为了加速他们各自所在国家的经济发展，热带雨林作为有价值的原木和其他资源正为人们所砍伐。
Ocean currents affect the climates of the lands near which they flow.
洋流影响其流经的附近地区的气候。
They explained the reason to us why they had hated us before.
他们向我们解释为什么他们不喜欢我们的原因。
非限定性定语从句非限定性定语从句在形式上就与主句很松散，它与主句之间有一个逗点"，"隔开；它对其先行词没有限定、修饰的作用，只起补充、说明的作用。有时也用它来对全句进行补充、说明。即使将其去掉，也不会影响句子意思。由于上述原因，非限定性定语从句在表达意思方面也有别于限定性定语从句。另外，非限定性定语从句在中文译文里，我们往往将其作为一个分句处理，而不把它作定语翻译。 例如：Earlier , the Babylonians had attempted to map the world , but they presented it in the form of a flattened disc rather than a sphere , which was the form adopted by Ptolemy .
（此句中，非限定性定语从句是对先行词a sphere进行补充、说明。）
更早之前，巴比伦人曾试图绘制世界地图，但是他们把它绘制成平盘状而不是托勒密所采用的球体状。
The combination of satellites, which transmit information , computers , which store information , and television , which displays information , will change every home into an education and entertainment center .
（此句中，三个非限定性定语从句分别对三个先行词：satellites ，computers和television进行补充、说明。如果去掉这三个非限定性定语从句，那么句子可简化为：
The combination of satellites , computers and television will change every home into an education and entertainment center .）卫星能传输信息，计算机能储存信息，电视能显示信息，把这些手段结合起来可以使每个家庭都成为教育娱乐的中心。
The sun heats the earth , which makes it possible for plants to grow .（此句中，非限定性定语从句是对全句进行补充、说明,将全句表达的意思看成"一件事情）。太阳给予大地热，这就使植物的生长成为可能。
The old man has a son , who is in the army .（此句中，非限定性定语从句是对先行词son进行补充、说明。但本句所传达的信息是："这位老人只有一个儿子" 。如果将此句改写成限定性定语从句：
The old man has a son who is in the army . 那位老人有一个在部队工作的儿子。那么，限定性定语从句就要对先行词son进行限定、修饰。这样一来，句子所传达的信息就变成了："这位老人有一个儿子在部队工作，还有其他的儿子在干别的工作"。）那位老人有一个儿子，他在部队工作。
补充：
掌握好定语从句的关系词很重要。
定语从句的关系词有很多，而且用法也很复杂，下面将几个关系词的"使用权限"进行归纳：
一、只能用that,不能用which, who 的几种情况:
1.先行词是人和物
The person and his dog that were killed in the accident were buried.
The man and his school that you told me are very interesting.
2.先行词是不定代词anything, something, everything, nothing, all, the one 等
Has everything that can be done been done?
All that you told me was difficult to understand.
3.先行词被最高级修饰
It is the longest film that I have seen in my life.
4.先行词被序数词修饰
It's the first time that I've been to Beijing.
The first stamp that was invented appeared in England.
5.先行词被just ,very, only等强调词语所修饰
It's the very house that I want to buy.
Only the vegetables that haven't been polluted are safe to eat.
二、只能用which, who,不能用that的两种情况:
1.非限制性定语从句
The Changjiang River, which is the third longest river in the world, lies in China.
Do you know Tom, who used to be the headmaster of this school?
2.整句话当先行词
It rained a lot, which made a flood happen.
He tore my photo, which made me angry.
三、只能用as,不能用which,that, who的两种情况:
1. 先行词被such, the same 修饰
I've never heard of such a terrible earthquake as you told me.
对比:It's such a terrible earthquake that many people were killed.
分析:定语从句的动词和先行词有动宾关系,如上面第一句应用as;结果状语从句应用such...that句型,表示"如此……以至于",如上面第二句。
2. 整句话当先行词,且从句有"正如……所知道/预料的一样"之意
Taiwan belongs to China, as we know.
As everybody can see, we have a good harvest in fruit this year, so the fruit will be cheap.
as与which以整句话当先行词的区别:1.which引导的句子有"这就使……"造成某些客观后果的意思,而as 引导的句子都含有主观认识的动词:如know, see, expect 之类.2. as 引导的定语从句可放在句首,这是唯一能放在先行词前的定语从句.当然,它也可放在先行词后.而which 却只能放在先行词后,不能放在句首.
四、that, which,who等关系代词在作定语从句中的宾语时可省略。如:
This is the house (that/ which) my grandparents built twenty years ago.
The person (who/ whom/ that) you just spoke to is our headmaster.
五、只能用when, where,不能用that, which的情况:
关系代词在从句中作状语.例如作live, work, study等不及物动词的状语.
试比较:I'll never forget the days that we spent together.
I'll never forget the days when we studied together.
He enjoyed the place that/ which he visited a week ago in Wuhan.
He enjoyed the place where he stayed a week ago in Wuhan.
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