非限制性定语从句只是对主句内容，或先行词的补充、解释或附加说明。主句与先行词或从句之间一般用逗号分开，常常单独翻译。没有它，主句意思仍然完整。引导非限制性定语从句的关系代词有as，which，who， whom，whose等，作定语从句的主语、宾语、表语及定语。关系副词有when，where等，作定语从句的状语。关系代词和关系副词在定语从句中一般不能省。
一、as引导非限制性定语从句时，可以代主句的全部或部分内容。常表说话人关于说话的依据、态度、评论、看法等。有“正如、像”等意思。定语从句可以置主句句首、句中或句末。as后常接expect，know，report，say，see等动词的主、被动语态句。
1． ____is known to everybody，the moon travels round the earth once every month．
A．It B．As C．That D．What
2．Mike，as we expected，attended the meeting．像我们所期望的那样，迈克参加了会议。
3．He wasn't unconsicious，as could be judged from his eyes．他并未失去知觉，这从他的眼神可以判断出来。
二、which引导非限制性定语从句的情况很多。
I．指代主句的全部或部分内容时，常表事实、状态、起因等，有“这就使得、这一点”等意思，常置主句末。主句与定语从句用逗号分开。
1．Dorathy was always speaking highly of her role in the play，，of course，madethe others unhappy．
A．who B．which C．this D．what
II．指代先行词有多种情况。定语从句置先行词后面。
1．在“n．／pron．／num．．．．＋prep．＋ which”，“prep．＋which”定语从句里。 1）They talked about a movie，the name of which I've never forgotten．他们谈论过一部电影，我决不会忘记片名。
2）In Sydney the Chinese team got 28 gold medals，16．5 of which were won by women．在悉尼奥运会，中国队夺得了28枚金牌，其中16．5枚是女子夺得的。
3）China has thousands of islands，the largest of which is Taiwan．中国有数千个岛屿，其中最大的是台湾岛。
4）Chaplin went to the states in 1910，by which time he had learnt to dance and act in comedies．卓别林1910年去了美国，那时他已学会跳舞和演喜剧了。
2．表唯一性，或者就是指代先行词的事物时。
The dam，which is the biggest in the world，is 3，830 metres long．大坝长3，830米，是世界上最大的坝。
3．先行词是独一无二的事物时。
The moon，which doesn't give out light itself，is only a satellite of the earth．月球本身不发光，它只是地球的一个卫星。
4．先行词表示类属的事物时。
Football，which is a very interesting game，is played all over the world．足球是一项非常有趣的运动，全世界都踢足球。
5．先行词是专有名词时。
1）Three of the biggest man-made projects in the world are the Great Wall of China，the Pyramids of Egypt and the Aswan High Dam， which is also in Egypt．世界上最大的三项人造工程是中国的长城，埃及的金字塔和阿斯旺高坝，它也在埃及。
2）The Nile，which used to flood every year，now runs more regularly below the dam．尼罗河年年涨洪水，现在比较正常地在大坝下面流过了。
6．先行词是表人的职业、品质、身份等名词，作定语从句的表语时。主句和定语从句之间含有对比的意思。
Mike's brother is a policeman，which he isn't．迈克的哥哥是警察，他可不是。
7．先行词是形容词作定语从句的表语时。主句和定语从句含有对比的意思。
Li Ling is very clever，which Li Long isn't．李玲很聪明，李珑可不是。
8．先行词是集体名词表整体意思时。
My family，which has 35 people，is a large one．我家有35口人，是一个大家庭。
9．先行词是国家名词表地域概念时。
Last year he went to Egypt，which is in Africa．他去年去了埃及，埃及在非洲。
三、as，which是指代主句内容的非限制性定语从句时，表依据、评论与表事实、状态等没有多大差别，又在主句末时，有时可以通用。
1．He is quite pleased，as ／which can be seen from his face．他非常高兴，从他的表情可以看出来。
2．You always work hard，as ／which everyone knows．大家都知道，你一向勤奋学习。
但是，上面两句把定语从句置主句句首时，就只能用as。
3．This elephant is like a snake，as anybody can see．像任何人都能看见那样，这头象像条蛇。
4．Bamboo is hollow，which makes it very light．竹子是空的，这就使得它很轻。 在否定意义的非限制性定语从句里，一般用which引导。
5．He came to my birthday party，which I didn't expect at all．我完全没有想到，他来参加我的生日聚会了。
四、who，whom，whose等引导非限制性定语从句时，指代人的普通名词、专有名词等。表示正是或专指先行词等情况。在定语从句中作主语、宾语、定语等。
1．Bob's father，who worked on the project，spent four years in Egypt．鲍勃的父亲从事这项工程，在埃及度过了四年。
2．Dad，this is Zheng Jie，who I knew in Paris．爸，这是郑杰，我在巴黎认识的。 这一句是用主格who代替宾格 whom。
3．His mother，whom he loved dearly， died in 1818．他非常爱戴他的母亲，她死于 1818年。
4．Mr King，whose legs were badly hurt， was quickly taken to hospital．金先生的腿受了重伤，他很快就被送去住院了。
n．／pron．／num．．．．prep．whom；prep． whom；in whose ＋n．；the ＋n．of whom等引导非限制性的定语从句。
5．He spoke of a pen-friend，the name of whom ／whose name I've never forgotten．他提到过一个笔友，他的名字我从未忘记。
6．About 2，000people，many of whom were Europeans，worked on the project．大约两千人从事过这项工程，其中很多是欧洲人。
7．In his office there are nine clerks，the youngest of whom is Mr Liu．他办公室有9个职员，其中最年轻的是刘先生。
8．I decided to write about Chaplin，one of whose films I had seen several years before．我决定写有关卓别林的文章，他的一部电影我几年前就看过了。
9．We went to hear this famous singer， about whom we had heard many stories．我们去听这个著名的歌唱家演唱。我们已经听说了有关他的很多故事。 品黄黑MYK
10．Mr Pattis，in whose office Ms King was working，was very surprised when he found out her secret．金女士在帕蒂思办公室工作。他发现她的秘密时感到非常惊奇。
五、when，where引导非限制性定语从句时，作定语从句的状语。when ＝and then， where ＝and there。why不引导非限制性定语从句。
1．He was quickly taken to hospital，where a doctor wanted to examine Mr King's legs．金先生很快被送去住院，在那里一个医生要检查他的腿。
2．We will put off the outing until next week，when we won't be so busy．我们把郊游推迟到下个星期，那时我们不会那么忙了。
3．He went to America in 1912，where and when he was noticed by an important film director．他在1912年去了美国，当时他在那里引起了一个重要电影导演的注意。
在prep．where ／when非限制性定语从句里，where ＝there，when ＝then。
4．His head soon appeared out of the second storey windows，from where he could see nothing but trees．他的头很快从第二层楼的一个窗户伸出来了，从那里他只能看见树木。
5．That was in 1929，since when things have been better．那是在1929年，从那时起情况就已经好转了。
6．I went to bed at ten，before when I was reading for an hour．我10点去睡觉，在那时以前我看了1小时的书。
有时候where ／when可以用prep．which替换。
7．The southern states wanted to set up a country of their own，where they would be free to keep black slaves．南方各州要建立一个属于他们自己的国家，在那里他们将自由地保留黑人奴隶。
8．I still remembered January 10，on which ／when he came to see me．我仍然记得1月10日，那一天他来看我了。
练习题：用所给的词语填空。少数可以用多次。
A．as B．which C．who D．whom E．whose F．when G．where H．by which time I．from which J．from whom K．north of which L．some of whom M ．the biggest of which
1．Dick is going to join in the football game，____was agreed at the meeting．
2．My brother had been an engineer，____ was what he wanted to be．
3．He reached Beijing in 1998，____， some time later，he became a professor．
4． ____is said above，the number of the students in our school has increased．
5．Write a letter to a friend，____has some difficulties with his studies．
6．There are four rooms in the house，____ is a drawing room．
7．We will move into the new house next week，____it will be completed．
8．When lost in work，he often was，____he would forget all about eating and sleeping．
9．He is a model worker，____we can learn a lot．
10．They had to delay the sports meet till next week，____the weather will be fine．
11．The workers，____stayed there for two years，came from Africa．
12．The building over there is a hospital，____ is a small river．
13．We shall have to make a decision about Ms King，____story I've just told you．
14．Mr Liu，____I spoke of just now， would go to Australia．
15．The Nile，____electricity is produced，now runs regularly below the dam．
Keys：1．A／B2．B3．G4．A（As）5．C6．M 7．H 8．A 9．J10．F 11．L 12．K 13．E 14．C ／D 15．I