1.先行词为all, anything, something, nothing, everything, much, little, none等不定代词时，关系代词一般只用that，不用which。在大多数情况下that可以省略.

Please tell me everything (that) you know about the matter.

2.as引出的限制性定语从句

在such …as的结构中as可作关系代词，引出限制性定语从句。有时和same连用，在从句中可作主语、宾语或表语等。

Such people as were recommended by him were reliable.

3.as引出的非限制性定语从句

as可作关系代词引出非限制性定语从句，代替整个主句，通常译为“(正)如…一样”，“(正)象…一样”等。as引导的从句一般用逗号与主句隔开，可以位于主句的前面、中间或后面.

I live a long way from work, as you know.

4.分隔式定语从句

定语从句一般紧跟在先行词之后，但有时会被其他句子成分与先行词隔开，从而构成分隔式定语从句。

The days are gone when power politics worked.

5.介词+关系代词(which/whom等)引出的定语从句

如果关系代词(which/whom等)在定语从句中作介词的宾语，那么这个介词可以提到从句前，构成“介词+关系代词(which/whom等)+定语从句”。在这种结构中关系代词不可以用that。

This is a subject about which we might argue for a long while.

6.关系代词在限制性定语从句中常可以省略,一般有以下几种情况：

1) 关系代词在从句中作直接宾语时，在大多数情况下可以省略。

This is something (that) you must always keep in mind.

关系代词作介词宾语时，介词位于句尾时关系代词可以省略;介词位于关系代词之前时则关系代词不能省略。

This is the room (which) Churchill was born in.

2) 关系代词作主语时一般不能省略，但如果关系代词在由there …be存在句构成的定语从句中作主语时常可省略。

This is the only book (that) there is on this subject.

3) 在way后面的定语从句中in which或that通常省略。

That’s the way (that/in which) I look at it.

7.非限制性定语从句

非限制性定语从句修饰先行词或整个主句，起补充说明的作用，如果省略掉，原句句意仍保持完整。非限制性定语从句要用逗号与主句隔开，其引导词不能用that。非限制性定语从句属于正式语体。经常考到的非限制性定语从句有以下三种。

1)由which、as引导的非限制性定语从句。which、as代表整个主句。

He said that he had never seen her before, which was not true.

2)由 “介词+关系代词”引导的非限制性定语从句。

He sent her a letter, in which he said that he was sorry for what he had done to her.

3)由 “数词、代词或名词+of +关系代词”引导的非限制性定语从句。

He had three sons, one of whom was my son’s classmate.