非限制性定語從句只是對主句內容，或先行詞的補充、解釋或附加說明。主句與先行詞或從句之間一般用逗號分開，常常單獨翻譯。沒有它，主句意思仍然完整。引導非限制性定語從句的關系代詞有as，which，who， whom，whose等，作定語從句的主語、賓語、表語及定語。關系副詞有when，where等，作定語從句的狀語。關系代詞和關系副詞在定語從句中一般不能省。
一、as引導非限制性定語從句時，可以代主句的全部或部分內容。常表說話人關于說話的依據、態度、評論、看法等。有“正如、像”等意思。定語從句可以置主句句首、句中或句末。as后常接expect，know，report，say，see等動詞的主、被動語態句。
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．像我們所期望的那樣，邁克參加了會議。
3．He wasn't unconsicious，as could be judged from his eyes．他并未失去知覺，這從他的眼神可以判斷出來。
二、which引導非限制性定語從句的情況很多。
I．指代主句的全部或部分內容時，常表事實、狀態、起因等，有“這就使得、這一點”等意思，常置主句末。主句與定語從句用逗號分開。
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．指代先行詞有多種情況。定語從句置先行詞后面。
1．在“n．／pron．／num．．．．＋prep．＋ which”，“prep．＋which”定語從句里。 1）They talked about a movie，the name of which I've never forgotten．他們談論過一部電影，我決不會忘記片名。
2）In Sydney the Chinese team got 28 gold medals，16．5 of which were won by women．在悉尼奧運會，中國隊奪得了28枚金牌，其中16．5枚是女子奪得的。
3）China has thousands of islands，the largest of which is Taiwan．中國有數千個島嶼，其中最大的是臺灣島。
4）Chaplin went to the states in 1910，by which time he had learnt to dance and act in comedies．卓別林1910年去了美國，那時他已學會跳舞和演喜劇了。
2．表唯一性，或者就是指代先行詞的事物時。
The dam，which is the biggest in the world，is 3，830 metres long．大壩長3，830米，是世界上最大的壩。
3．先行詞是獨一無二的事物時。
The moon，which doesn't give out light itself，is only a satellite of the earth．月球本身不發光，它只是地球的一個衛星。
4．先行詞表示類屬的事物時。
Football，which is a very interesting game，is played all over the world．足球是一項非常有趣的運動，全世界都踢足球。
5．先行詞是專有名詞時。
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．世界上最大的三項人造工程是中國的長城，埃及的金字塔和阿斯旺高壩，它也在埃及。
2）The Nile，which used to flood every year，now runs more regularly below the dam．尼羅河年年漲洪水，現在比較正常地在大壩下面流過了。
6．先行詞是表人的職業、品質、身份等名詞，作定語從句的表語時。主句和定語從句之間含有對比的意思。
Mike's brother is a policeman，which he isn't．邁克的哥哥是警察，他可不是。
7．先行詞是形容詞作定語從句的表語時。主句和定語從句含有對比的意思。
Li Ling is very clever，which Li Long isn't．李玲很聰明，李瓏可不是。
8．先行詞是集體名詞表整體意思時。
My family，which has 35 people，is a large one．我家有35口人，是一個大家庭。
9．先行詞是國家名詞表地域概念時。
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是指代主句內容的非限制性定語從句時，表依據、評論與表事實、狀態等沒有多大差別，又在主句末時，有時可以通用。
1．He is quite pleased，as ／which can be seen from his face．他非常高興，從他的表情可以看出來。
2．You always work hard，as ／which everyone knows．大家都知道，你一向勤奮學習。
但是，上面兩句把定語從句置主句句首時，就只能用as。
3．This elephant is like a snake，as anybody can see．像任何人都能看見那樣，這頭象像條蛇。
4．Bamboo is hollow，which makes it very light．竹子是空的，這就使得它很輕。 在否定意義的非限制性定語從句里，一般用which引導。
5．He came to my birthday party，which I didn't expect at all．我完全沒有想到，他來參加我的生日聚會了。
四、who，whom，whose等引導非限制性定語從句時，指代人的普通名詞、專有名詞等。表示正是或專指先行詞等情況。在定語從句中作主語、賓語、定語等。
1．Bob's father，who worked on the project，spent four years in Egypt．鮑勃的父親從事這項工程，在埃及度過了四年。
2．Dad，this is Zheng Jie，who I knew in Paris．爸，這是鄭杰，我在巴黎認識的。 這一句是用主格who代替賓格 whom。
3．His mother，whom he loved dearly， died in 1818．他非常愛戴他的母親，她死于 1818年。
4．Mr King，whose legs were badly hurt， was quickly taken to hospital．金先生的腿受了重傷，他很快就被送去住院了。
n．／pron．／num．．．．prep．whom；prep． whom；in whose ＋n．；the ＋n．of whom等引導非限制性的定語從句。
5．He spoke of a pen-friend，the name of whom ／whose name I've never forgotten．他提到過一個筆友，他的名字我從未忘記。
6．About 2，000people，many of whom were Europeans，worked on the project．大約兩千人從事過這項工程，其中很多是歐洲人。
7．In his office there are nine clerks，the youngest of whom is Mr Liu．他辦公室有9個職員，其中最年輕的是劉先生。
8．I decided to write about Chaplin，one of whose films I had seen several years before．我決定寫有關卓別林的文章，他的一部電影我幾年前就看過了。
9．We went to hear this famous singer， about whom we had heard many stories．我們去聽這個著名的歌唱家演唱。我們已經聽說了有關他的很多故事。 品黃黑MYK
10．Mr Pattis，in whose office Ms King was working，was very surprised when he found out her secret．金女士在帕蒂思辦公室工作。他發現她的秘密時感到非常驚奇。
五、when，where引導非限制性定語從句時，作定語從句的狀語。when ＝and then， where ＝and there。why不引導非限制性定語從句。
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，在那里一個醫生要檢查他的腿。
2．We will put off the outing until next week，when we won't be so busy．我們把郊游推遲到下個星期，那時我們不會那么忙了。
3．He went to America in 1912，where and when he was noticed by an important film director．他在1912年去了美國，當時他在那里引起了一個重要電影導演的注意。
在prep．where ／when非限制性定語從句里，where ＝there，when ＝then。
4．His head soon appeared out of the second storey windows，from where he could see nothing but trees．他的頭很快從第二層樓的一個窗戶伸出來了，從那里他只能看見樹木。
5．That was in 1929，since when things have been better．那是在1929年，從那時起情況就已經好轉了。
6．I went to bed at ten，before when I was reading for an hour．我10點去睡覺，在那時以前我看了1小時的書。
有時候where ／when可以用prep．which替換。
7．The southern states wanted to set up a country of their own，where they would be free to keep black slaves．南方各州要建立一個屬于他們自己的國家，在那里他們將自由地保留黑人奴隸。
8．I still remembered January 10，on which ／when he came to see me．我仍然記得1月10日，那一天他來看我了。
練習題：用所給的詞語填空。少數可以用多次。
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