定语从句有限制性和非限制性定语从句两种。一般地说，限制性定语从句用来限制先行词的意义，与先行词关系密切，是整个句子不可缺少的一部分，否则会造成全句意义不明确。而非限制性定语从句带有补充说明的性质，与先行词之间存在一种松散的修饰关系，去掉后一般不会影响句子的主要意义。概括起来，两者有如下区别：
1）形式不同：限制性定语从句常紧跟其先行词，中间不用逗号隔开；而非限制性定语从句与先行词之间常用逗号隔开。
After living in Paris for fifty years, he returned to the small
town where he grew up as a child. (NMET 1996)
在巴黎生活了50年后，他回到了小时候生活过的那个小镇。
2）意义不同
请比较下面两个句子：
My brother who is 18 years old is a college student.
我18岁的那个弟弟是大学生。
My brother, who is 18 years old, is a college student.
我弟弟是大学生,今年18岁。
第一句中，限制性定语从句限制了先行词的意义，明确是18岁的那个弟弟（而不是其他的，即有可能还有几个弟弟）。第二句使用了非限制性定语从句，表明可能只有一个弟弟，１８岁，起补充说明作用。
另外，如果先行词是专有名词或独一无二的人或物，通常用非限制性定语从句。
His mother, who loved him very much, died in 1818.
最爱他的母亲于1818年去世了。
I like to chat with Jack, who is a clever boy. 我喜欢和杰克聊天，他是个聪明人。
When he was 17, he went to a technical school in Zurich,
Switzerland, where he studied maths and physics.
17岁那年，他到瑞士的苏黎士一家技校上学，在那儿他学习了数学和物理。
3）先行词不同：限制性定语从句的先行词通常是名词或代词，而非限制性定语从句的先行词除名词或代词外，还可以是整个主句（此时，常用which或as引导从句）。
All that is needed is a supply of oil. (MET1989) 所需要的只是一些油而已。
Carol said the work would be done by October, which personally I
doubted very much. (NMET 1999) 凯罗尔说工作可在10月前完工，这一点我个人深感怀疑。
As has been announced, we shall have our final exams next week.
(2003上海春) 我们将于下周举行期末考试，这一点刚才已经宣布了。
句中as 是关系代词，引导非限制性定语从句，指代后面整个主句的内容。
4）引导词不同：限制性定语从句的引导词可用关系代词that, which, who( whom, whose),
as和关系副词when, where, why；除that 和why 外，其它的关系代词、关系副词均可引导非限制性定语从句。
He is the only one of the students who / that has been a winner
of scholarship for three years. (NMET 2000春)
他在这些学生中是唯一连续3年获得奖学金的一个。
She heard a terrible noise, which brought her heart into her
mouth. (MET 1991) 她听到一个可怕的声音，差点儿把胆都给吓破了。（句中which不可用that代替）
The famous basketball star, who tried to make a comeback,
attracted a lot of attention. (2002 春季)
那个篮球明星试图复出江湖，吸引了众人的注意。（句中who不可用that代替）
5）引导词省略情况不同：众所周知，在限制性定语从句中，关系代词如果充当从句的宾语可以省略；而非限制性定语从句中的关系代词是不能省略的。
Is this the book (that / which) you are looking for? (NMET 1999)
这就是你要找的那本书吗？（关系代词that / which充当从句的宾语，可以省略）
The result of the experiment was very good, which we hadn't
expected. 实验的结果非常好，这简直出乎我们的意料。( 关系代词
which在从句中充当expect的宾语，但因为引导的是非限制性定语从句，所以不能省略)
6）译法不同：把句子翻译成汉语时，限制性定语从句中，通常先译从句后译先行词；而非限制性定语从句中则往往相反。
We are living in an age when many things are done on computer.
(2003 北京春) 我们生活在一个许多工作都由电脑来完成的时代。
We will be shown around the city: school, museums and some other
places, where other visitors seldom go. (2002北京)
我们将被带去参观全市各地：包括学校、博物馆和其它场所，那些其它游客很少去的地方。
巩固练习
1. The doctor was very impolite to the patient, ________ of
course, made things even worse.
A. who B. whom C. which D. what
2. He's got himself into a dangerous situation ________ he is
likely to lose control over the plane.
A. where B. which C. while D. why
3. I'd like a car ________ front lights are big and round.
A. which B. who C. whose D. of which
4. David gets up early and takes a walk in the morning, ________
is usual with him.
A. which B. that C. as D. such
5. He will have to put off his concert until next month,
________ he finishes his travel around the world.
A. which B. where C. when D. that
6. I'll never forget the days ________ I lived in the country
with the farmers, ________ have a great effect on me.
A. that; which B. when; which C. which; that D. when; it
7. The weather turned out to be very good, ________ was more
than we could expect.
A. what B. which C. that D. this
8. Charlie Chaplin, for ________ life was hard, began acting at
the age of five.
A. who B. whom C. whose D. his
9. We are talking about the subject ________ importance I
haven't realized yet.
A. which B. that C. of which D. whose
10. The science of medicine, ________ progress has been very
rapid, is perhaps the most important of all the sciences.
A. which B. that C. in which D. with which
Key:
1-5 CACCC 6-10 BBBDC
