在复合句中，修饰某一名词或代词的从句叫做定语从句。被定从句修饰的词叫做先行词。定语从句必须放在先行词之后。引导定语从句的关联词有关系代词和关系副词。例如： This is the present that he gave me for my birthday.
Do you know everybody who came to the party?
I still remember the night when I first came to the village?
This is the place where Chairman Mao once lived.
1. who 指人，在定语从句中作主语。例如：
The person who broke the window must pay for it.
The boy who is wearing the black jacket is very clever.
A dictionary is a book which gives the meaning of words.
Here is the book (which) the teacher mentioned yesterday.
I've read the newspaper that(which) carries the important news.
Who is the person that is reading the newspaper over there?
1. 只能使用that,不用which 的情况：
(1) 先行词是all, few, little, nothing, everything, anything 等不定代词时。例如： All that he said is true.
(2) 先行词被only, no, any, all,等词修饰时。例如：
He is the only foreigner that has been to that place.
He was the second (person) that told me the secret.
This is the best book (that) I have read this year.
He talked about the people and the things he remembered.
2. 只能用which，不用that 的情况：
The meeting was put off, which was exactly what we wanted.
The thing about which he is talking is of great importance.