1、which、who、whom、whose、that、as等。
这些词可以引导非限制性定语从句，用来对前面的名词或代词进行补充说明，但不是必不可少的信息。
例如，The book, which was written by my favorite author, is my favorite.（这本书是我最喜欢的，它是由我最喜欢的作家写的。
）其中which引导的从句是非限制性定语从句，用来补充说明前面的名词the book。
非限定性定语从句的分类包括由who引导、由whom引导、由whose引导、由which引导、由when引导、由where引导、由as引导等，非限定性定语从句放在句子中间时，其前后都需要使用逗号隔开。
当先行词表人时，用who，whom, whose来引导非限制性定语从句。
第一， who引导非限制性定语从句时，一般在从句中作主语成分。
【例】My father, who is a English teacher.
第二， whom引导非限制性定语从句时，一般在从句中作动词宾语或介词宾语。当它作介词宾语时，介词可以放在句尾。
【例】My father, from whom I have learned a lot, is an excellent English teacher.
【例】Tom, whom I met in Shanghai, is now back in Shenzhen.
第三， whose引导非限定性定语从句时，因为它是who的所有格形式，所以它在从句中一般作定语。
【例】Tom, whose mother is a nurse, studies very hard.
注意，whose还可以指物。后面会讲解。

当先行词表物时，用which， whose来引导非限制性定语从句。
第一， which引导非限制性定语从句时，它在从句中常作主语、动词宾语、介词宾语、表语。
在以下几种情况下，要用which引导非限制性定语从句。
1． 在先行词表唯一或独一无二的事物时。如the moon, the sun, the earth等
2． 先行词表类属的事物时。如football是game的一种
3． 先行词表某人的职业、品质、身份等，引导词在从句中作表语，并且主句与从句是对比关系时。
【例】Tom’s father is a teacher, which he is not.
4． 当先行词是某专有名词时。如Great Wall，Egypt等
5． 先行词是形容词，引导词在从句中作表语，并且主句与从句是对比关系时.
【例】Tom is very smart, which Ada is not.
6． 先行词是集体名词表整体时。如family等
7． 先行词是国家名词表地域时。如Egypt表Africa
注意，which除了修饰名词、名词性短语之外，它还可以修饰形容词、从句和整个主句。是所有关系代词与关系副词中唯一一个有这个功能的。
【例】Tom was very patient towards his kids which his wife seldom was. 修饰形容词patient。
【例】When deeply absorbed in study, which Tom often was, he would forget everything. 修饰整个主句。
【例】Tom said that he had never seen me before, which was not true. 修饰从句。
第二，whose引导非限制性定语从句并指物时，这个物可以是动物，也可以是没有生命特征的事物。
【例】The movie, whose style is rigidly formal, is typical of the period.
当先行词表时间或地点时，用关系副词when, where引导非限制性定语从句。
第一，when引导非限制性定语从句时，它在从句中是作时间状语。
【例】Tom will put off the picnic until September 7th, when he will be free.
第二，where引导非限制性定语从句时，它在句中是作地点状语。
【例】We went to Shanghai, where we lived for one week.
注意，如文章开头所说的，当要表达疑问和原因的非限制性定语从句时，不能用why来引导，而是用for which来代替。
【例】Tom had told them the reason, for which he did not join the travel.

当先行词是整个句子时，使用as, which来引导非限制性定语从句。
第一，当as引导非限定性定语从句时，是对主句起说明作用。As相当于关系代词，它在从句中一般作主语、宾语、表语。它一般有一些固定的用法与搭配，后面常接expect, know, report, say, see等动词的主动或被动语态，如as it is know, as is reported in the newspaper, as always mentioned above, as is usual, as is often the case, as is know to all, as we all know等等。
as可放在主句之前，主句之后，主句中间。
【例】As we all know, the basketball is round. 放在句首，在从句中作宾语。
【例】I forgot to bring my handbag with me, as was often the case. 放在主句之后，在从句中作主语。
【例】Kid as she was, she was chosen leader of the team. 放在主句之中，在从句里作表语。
