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、在蒉翟蛳庹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，不做任淡馋衔锂何成份，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、从定凶及淄靥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 re艘早祓胂ceived 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。）