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一､区别
定语从句就其与先行词的关系而言，可分为限制性定语从句和非限制性定语从句，两者之间有如下区别:
1.限制性定语从句在意义上与先行词关系密切，是不可缺少的定语，如果去掉，主句的意思就不完整或失去意义。从句和主句的关系十分密切，写时不用逗号分开，关系代词有时可以省略。例如:
That is the girl who came to see you this morning.
那就是今天上午来看你的女孩。
The Great Wall is a place (that) I’ve always wanted to travel.
长城是我一直想参观的一个地方。
2.非限制性定语从句和主句的关系不十分密切，只是对先行词作些附加的说明，如果去掉，主句的意思仍然清楚;从句和主句之间往往用逗号分开。用来引导非限制性定语从句的有关系代词which， who， whom， whose和关系副词where， when这些，而that一般不用来引导非限制性定语从句。例如:
Yesterday I visited Mr Wang， who looked very tired.
昨天我拜访了王先生，他看上去很疲倦。
Mum bought me a computer as my birthday present， which was just what I longed for.
妈妈给我买了台电脑作为生日礼物，这正是我所渴望的。
3.比较:
He has two sisters who are studying abroad.(限制性定语从句:He has more than two sisters.) 他有两个在国外留学的姐姐。
He has two sisters， who are studying abroad.(非限制性定语从句:He has only two sisters.) 他有两个姐姐，她们在国外留学。
She will wear no clothes which will make her different from others.
她不会穿一些与众不同的衣服。
She will wear no clothes， which will make her different from others.
她不穿衣服，这会使她显得与众不同。
二､非限制性定语从句引导词的选择
1.关系代词的选择
(1)非限制性定语从句的关系代词不能用that。指人的，如引导词作主语用who，作宾语用whom;指物时要用which。即使引导词作宾语也不能省略。例如:
Mr Zhang， who came to see me yesterday， is an old friend of my
father’s. 昨天来看我的张先生，他是我父亲的一位老朋友。(作主语)
(2)as和which的选择
as和which都可用来引导非限制性定语从句，来指代整个主句的内容。非限制性定语从句位于句末且as或which在从句中作主语､宾语或表语时，两者常可互换。例如:
He failed in the exam， which/as was natural.
他考试没及格，这很自然。(作主语)
He is an honest boy， which/as anybody can see.
任何人都能看得出来，他是个诚实的孩子。(作宾语)
The man seemed a German， which/as in fact he was.
那个人似乎是个德国人，事实上他就是德国人。(作表语)
但要注意以下区别:
①如果非限制性定语从句位于主句之前或插在主句之中时，只能用as。例如:
As we have seen， oceans cover more than 70 percent of the earth.
我们已经看到，海洋占地球面积的百分之七十多。
This kind of computer， as is well-known， is out of date.
众所周知，这种计算机现在已经过时了。
②as在从句中作主语，后面常跟行为动词的被动式;which在从句中作主语，后常跟行为动词的主动式。例如:
She has been late again， as was expected.
她又迟到了，这是预料之中的。
It rained hard yesterday， which prevented me from going to the park.
昨天雨下得很大，使我无法去公园。
③当从句和主句语意一致时用as，表“正如”;反之用which。例如:
The thief came again， as was expected.
小偷又一次来了，这是预料之中的。(语意一致)
The elephant is like a snake， which is not right.
大象像一条蛇， 这是不对的。(语意不一致)
④as常用在 as often happens， as was pointed out， as was said above， as I remember， as I understand等结构中。例如:
Jack has won the first prize， as often happens.
像往常一样，杰克获得了一等奖。
2.关系代词和关系副词的选择
在非限制性定语从句中，关系代词和关系副词的选择方法和限制性定语从句一样:引导词在从句中作主语､宾语､表语､定语时，用关系代词;作状语时用关系副词。例如:
I want to buy the house， which has a garden.
我想买那个房子，那个房子有一个花园。(作主语)
The place， which I visited before， has changed greatly.
这地方我以前来过，现在变化很大。(作宾语)
The story in the book takes place in Canada， where Doctor Bethune was born.
书中故事发生在加拿大， 白求恩大夫就是在那里出生的。(作状语)
3.关系代词､人称代词和指示代词的选择
选关系代词还是人称代词，关键是分清句子结构。如果是非限制性定语从句(两句中间用逗号隔开，且无连词)，用关系代词; 如果是并列句(全句中有连词，两句中间用句号或分号)，用人称代词或指示代词。例如:
He has three sons， none of whom is a doctor.
He has three sons， who are doctors.
He has three sons， but none of them is a doctor.
He has three sons; they are doctors.
He has three sons. They are doctors.
定语从句就其与先行词的关系而言，可分为限制性定语从句和非限制性定语从句，两者之间有如下区别:
1.限制性定语从句在意义上与先行词关系密切，是不可缺少的定语，如果去掉，主句的意思就不完整或失去意义。从句和主句的关系十分密切，写时不用逗号分开，关系代词有时可以省略。例如:
That is the girl who came to see you this morning.
那就是今天上午来看你的女孩。
The Great Wall is a place (that) I’ve always wanted to travel.
长城是我一直想参观的一个地方。
2.非限制性定语从句和主句的关系不十分密切，只是对先行词作些附加的说明，如果去掉，主句的意思仍然清楚;从句和主句之间往往用逗号分开。用来引导非限制性定语从句的有关系代词which， who， whom， whose和关系副词where， when这些，而that一般不用来引导非限制性定语从句。例如:
Yesterday I visited Mr Wang， who looked very tired.
昨天我拜访了王先生，他看上去很疲倦。
Mum bought me a computer as my birthday present， which was just what I longed for.
妈妈给我买了台电脑作为生日礼物，这正是我所渴望的。
3.比较:
He has two sisters who are studying abroad.(限制性定语从句:He has more than two sisters.) 他有两个在国外留学的姐姐。
He has two sisters， who are studying abroad.(非限制性定语从句:He has only two sisters.) 他有两个姐姐，她们在国外留学。
She will wear no clothes which will make her different from others.
她不会穿一些与众不同的衣服。
She will wear no clothes， which will make her different from others.
她不穿衣服，这会使她显得与众不同。
二､非限制性定语从句引导词的选择
1.关系代词的选择
(1)非限制性定语从句的关系代词不能用that。指人的，如引导词作主语用who，作宾语用whom;指物时要用which。即使引导词作宾语也不能省略。例如:
Mr Zhang， who came to see me yesterday， is an old friend of my
father’s. 昨天来看我的张先生，他是我父亲的一位老朋友。(作主语)
(2)as和which的选择
as和which都可用来引导非限制性定语从句，来指代整个主句的内容。非限制性定语从句位于句末且as或which在从句中作主语､宾语或表语时，两者常可互换。例如:
He failed in the exam， which/as was natural.
他考试没及格，这很自然。(作主语)
He is an honest boy， which/as anybody can see.
任何人都能看得出来，他是个诚实的孩子。(作宾语)
The man seemed a German， which/as in fact he was.
那个人似乎是个德国人，事实上他就是德国人。(作表语)
但要注意以下区别:
①如果非限制性定语从句位于主句之前或插在主句之中时，只能用as。例如:
As we have seen， oceans cover more than 70 percent of the earth.
我们已经看到，海洋占地球面积的百分之七十多。
This kind of computer， as is well-known， is out of date.
众所周知，这种计算机现在已经过时了。
②as在从句中作主语，后面常跟行为动词的被动式;which在从句中作主语，后常跟行为动词的主动式。例如:
She has been late again， as was expected.
她又迟到了，这是预料之中的。
It rained hard yesterday， which prevented me from going to the park.
昨天雨下得很大，使我无法去公园。
③当从句和主句语意一致时用as，表“正如”;反之用which。例如:
The thief came again， as was expected.
小偷又一次来了，这是预料之中的。(语意一致)
The elephant is like a snake， which is not right.
大象像一条蛇， 这是不对的。(语意不一致)
④as常用在 as often happens， as was pointed out， as was said above， as I remember， as I understand等结构中。例如:
Jack has won the first prize， as often happens.
像往常一样，杰克获得了一等奖。
2.关系代词和关系副词的选择
在非限制性定语从句中，关系代词和关系副词的选择方法和限制性定语从句一样:引导词在从句中作主语､宾语､表语､定语时，用关系代词;作状语时用关系副词。例如:
I want to buy the house， which has a garden.
我想买那个房子，那个房子有一个花园。(作主语)
The place， which I visited before， has changed greatly.
这地方我以前来过，现在变化很大。(作宾语)
The story in the book takes place in Canada， where Doctor Bethune was born.
书中故事发生在加拿大， 白求恩大夫就是在那里出生的。(作状语)
3.关系代词､人称代词和指示代词的选择
选关系代词还是人称代词，关键是分清句子结构。如果是非限制性定语从句(两句中间用逗号隔开，且无连词)，用关系代词; 如果是并列句(全句中有连词，两句中间用句号或分号)，用人称代词或指示代词。例如:
He has three sons， none of whom is a doctor.
He has three sons， who are doctors.
He has three sons， but none of them is a doctor.
He has three sons; they are doctors.
He has three sons. They are doctors.