定语有限制性和非限制性两种。限制性定语从句是先行词不可缺少的部分非定语从句，去掉它主句意思往往不明确；非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，例如：
This is the house which we bought last month. 这是我们上个月买的那幢房子。（限制性）
The house, which we bought last month, is very nice.这幢房子很漂亮，是我们上个月买的。（非限制性）
2) 当先行词是专有名词或物主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。
3) 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。
说明：关系代词that和关系副词why不能引导非限制性定语从句。
非限制性定语从句只与先行词有一种松散的修饰关系，在口语中用停顿的方法表示，在书面语中用逗号分开。因此从句中的关系代词和关系副词不能省略。that一般不引导描述性定语从句。在非限制性定语从句中，所用的关系代词是：who、whom指人，which指物，whose表示所属；关系副词是：when指时间，where指地点。如：
I like to chat with John, who is a clever fellow. 我喜欢与约翰交谈，他是个聪明人。
Water, which is a clear liquid, has many uses. 水是一种清澈的液体，有许多用途。
Once more I am in Boston, where I have not been for ten years. 我又一次来到了波士顿，我有十年没有到这里来了。
非限制性定语从句形式上是从句，其功能实质上相当于一个分句。如：
Then he met Mary, who invited him to a party. 后来他遇到玛丽，玛丽邀请他去参加晚会。(who实际上=and she)
When he was seventeen he went to a technical school in Zurich, Switzerland, where he studied mathematics and physics. 他17岁时，到瑞士苏黎世一专科学校上学，他在那里学习数学和物理学。(where=and there)
有时非限制性定语从句的含义相当于一个状语从句。如：
We don’t like the room, which is cold. 我们不喜欢那个房间，它很冷。(which is cold=since it is cold)
He said he was busy, which was untrue. 他说他很忙，其实不然。(which was untrue=though it was untrue)
I want him, who knows some English. 我要他，他懂得些英语。(who knows some English=for he knows some English)