一、特点：
1.它只对所修饰的词或部分作进一步的解释或说明。
2.通常都用逗号隔开。
3.关系代词中（与限制性相比）已没有that一词；
关系副词中（与限制性相比）已没有why一词。
二、引导非限制性定语从句的关系代词有：
who, whom, whose, which.
This note was left by Mr．Li, who was here a moment ago.
这个条子是李先生留的，他刚才到这儿来过。
Mr．Smith , whom I often visit (call on) , is my good friend.
史密斯先生是我的好朋友，我常去拜访他。
The latest model of this car, whose exterior dimensions remain unchanged, gives much more passenger room.
最新型的这种小汽车，外部尺寸没有变化，但乘坐空间却大多了。
China has hundreds of islands, the largest of which is Taiwan.
中国有数以百计的岛屿，其中最大的是台湾。
三、引导非限制性定语从句的关系副词有：
when和where
In the old days, when I was a little boy, the city had no industry.
在过去，我还小时，这个城市没有工业。
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星期，那时我们就不会这儿忙了。
She is going to spend the summer holiday in Tsingtao, where she has some relatives.
她准备到青岛过暑假，那儿她有一些亲属。
They will fly to Kunming, where they plan to stay for two or three days, and then go on to Kweilin.
他们将乘飞机到昆明，在那儿计划待两三天，然后去桂林。
四、在这种从句中，which有时可代表前面说的整个情况或某一部分，而不是代表一个词：
I lost my way, which delayed me considerably.
我迷了路，这使我耽搁了很久。
He failed his exam， which proves that he wasn‘t working hard enough.
他考试没及格，这证明他不够用功。
