1、1.先行词不同
定语从句的先行词是名词或代词，而同位语从句的先行词是抽象名词如news，idea，fact，suggestion等。
This is the pen that he bought yesterday.
(定语从句，先行词为pen。）
The news that Tom would go abroad is told by him.
(同位语从句，先行词为抽象名词news。）
2、2.在从句中是不是做成分
that在定语从句中做成分，做主语，宾语，表语，做宾语时可以省略。而在同位语从句中仅起连接作用，不做任何成份，不能省略。
The news (that) he told me yesterday was true.
(定语从句，that在从句中做told的宾语，可以省略。）
The news that our team has won the game was true.
(同位语从句，that在从句中不做成份，不能省略。）
3、3.从定义上区分
定语从句中的that是对先行词的修饰和限制。
同位语从句中的that是对抽象名词的解释和说明。
The order that we received yesterday was that we should send a few people tohelp the other groups.
(定语从句，that从句是修饰先行词order的。）
The order that we should send a few people to help the other groups was received yesterday.
(同位语从句，that从句是对order进行解释说明。）
