非限定性定语从句的作用是对所修饰的成分作进一步说明，通常和主句间用逗号隔开，将从句拿掉后其他部分仍可成立

1. which引导的非限定性定语从句来说明前面整个句子的情况或主句的某一部分 ，that不能用于引导非限制性定语从句

例如:

Her house,which was built a hundred years ago,stood still in the earthquake.(那座房子在地震中依然耸立，它是一百多年前建造的。)

3. 有时as也可用作关系代词

4. 在非限定性定语从句中，用who, whom代表人，用which代表事物.

5.当先行词是专有名词或物主代词和指示代词所修饰时，其后的定语从句通常是非限制性的。

例如：

Charles Smith, who was my former teacher, retired last year. 查理·史密斯去年退休了，他曾经是我的老师。

My house, which I bought last year, has got a lovely garden.　我去年买的的那幢房子带着个漂亮的花园。

This novel, which I have read three times, is very touching. 这本小说很动人，我已经读了三遍。

6.非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，

例如：

He seems not to have grasped what I meant, which greatly upsets me. 他似乎没抓住我的意思，这使我心烦。

Liquid water changes to vapor, which is called evaporation. 液态水变为蒸汽，这就叫做蒸发。

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。