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)