定语从句（Attributive Clauses），在句中作定语，其作用是修饰主句的某个名词性成分，相当于形容词，所以又称为形容词性从句。通常定语从句皆置于它所修饰的名词或代词之后，被修饰的名词或代词叫做先行词（antecedent），而引导定语从句的词称为关联词。
This is the engineer who he cooperated with in the project.
先行词指人时，关系代词既可以用who, 也可以用that。但关系代词在定语从句中用作主语时，多用主格who。如：
★ Beyond the practical need to make order out of chaos, the rise of the dictionaries is associated with the rise of the English middle class, who were anxious to define and circumscribe the various world to conquer.（persons, those, people等先行词多用关系代词who）
★ It is Tom who should be punished.（在强调结构中，指人时多用who）
★ The American zoologist Donald Griffin, who was largely responsible for the discovery of sonar in bats, coined the term ‘echolocation’ to cover both sonar and radar, whether used by animals or by human instruments.
朗阁海外考试研究中心分析认为，在下列情况中多用that, 而不用who或者which。如：
★ much （很多）
little （很少）
none （没有任何）
everything （任何事情） +先行词 +that
everybody （任何人）
nothing （没有任何）
nobody （没有人）
E.g. There is nothing in the world that can frighten him.
★ 先行词前面有*形容词修饰时
He is the most boring speaker that I have ever heard.
This is the most interesting book that I have ever read.
★ 先行词有以下代词或者有下列代词修饰时
the first
the last
the only
the same +先行词 +that
the very
all
any
no
every
E.g. This is the last chance that you have.
This is the very thing that I lost yesterday.
This is the first thing that he wants to do.
You are the only friend that we have.
He is the last person that Tom wants to see.
He has told the police all that he knew.
★ 前面已经有疑问词who或者which时
E.g. Who that knows him will trust him?
Which of these trains is the one that goes to Nanjing?
Which is the bike that you have lost?
★ 兼指人和物/事物时
E.g. The man and the horse that fell into the river were drowned.
Look at the girl and her dog that are crossing the bridge.
在下列情况中则多用which。如：
★ 离先行词较远时
He told me the story of the craftsman which I narrated at the beginning of this book.
★ those + 复数名词 之后
You should buy one of those clothes which sell best.
★ which
