(1) 定语从句中的that既代替先行词,同时以在从句中作某个成分(主语或宾语),而同位语从句中的that是连词,只起连接主句与从句的作用,不充当句中任何成分。
(2) 定语从句是形容词性的,其功能是修饰先行词,对先行词加以限定,描述定的性质或特征;同位语从句是名词性的,其功能是对名词进行补充说明。例如:
1) The news that he told me is that Tom would go abroad next year.(他告诉我的消息是汤姆明年将出国。)(第一个that引导的从句是定语从句,that在从句中作宾语)
2)The news that Tom would go abroad is told by him.(汤姆将出国的消息是他讲的。)(同位语从句,that在句中不作任何成分)