区分同位语从句和定语从句，有以下几种途径：1.意义的不同同位语从句是对名词加以补充说明，它与被修饰词语通常可以划等号；而定语从句是对名词进行修饰和限定，它的作用是将所修饰的名词与其他类似的东西区别开来：We are glad at the news that he will come.听到他要来这个消息我们很高兴。(news的内容就是that he will come，故that引导的是同位语从句)We are glad at the news that he told us.听到他告诉我们的这个消息我们很高兴。(that从句是限制the news的内容的，即我们高兴只是因为他告诉的这个news而不是其他的news，故that从句为定语从句)2.被修饰词语的区别定语从句的先行词是名词或代词；而同位语从句的先行词只能是名词，而且仅限于idea，plan，fact，theory，promise，hope，news，doubt，truth，information，suggestion，question，thought，belief，conclusion等少数抽象名词。如：The boy who is playing football is my classmate．(定语从句，boy不能跟同位语从句）Those who work hard will succeed．(定语从句，代词those不能跟同位语从句）The fact that he had not said anything surprised everybody．(同位语从句）The fact that you are talking about is important．(同位语从句）3．when，where，why引导的定语从句的先行词一定分别是表示时间、地点和原因的名词，而三者引导的同位语从句的先行词则肯定不是表示时间、地点和原因的名词。

如：WhenI will never forget the day when I joined the army.(定语从句,when作状语,其先行词是表时间的词）We have no idea when she was born.(同位从句，when 做状语，其先行词不是表时间的词)WhereThis is the house where I lived two years ago.(定语从句，where作状语，其先行词是表地点的名词house)He put forward to the question where the meeting would be held.(同位语从句，where做状语，其前的名词the question不表地点)WhyThis is the reason why she will not attend the meeting.(定语从句，why作状语，其前的名词是reason）The teacher had no idea why Jack was absent.(同位语从句 why作状语，其前的名词不表原因)4．同位语的名词与从句没有逻辑关系，而定语从句，先行词在从句中是逻辑上的主语，宾语，表语等。

The teacher had no idea why Jack was absent.(同位语从句，idea在其后的从句中不做任何成分)It is a question how he managed to solve the mystery.(同位语从句,question在其后的从句中不做成分）We have some doubt whether they can complete the task on time. (同位语从句,doubt在其后的从句中不做成分）I will never forget the day when I joined the army.(定语从句,其先行词在从句中作状语）This is the house where I lived two years ago.(定语从句,where作状语,先行词在从句中作状语)The news that you told me yesterday was really exciting. (定语从句，其先行词news在从句中作宾语)5．引导同位语从句的连词不可以省略，而引导定语从句的关系代词作宾语时常可省略。The news that he is our monitor is true.他是我们班长的消息是真的。The news (that) he told is exciting.他说的消息令人兴奋。