一、从句法功能上来看

同位语从句中连接词that在从句中不作句子成分，也没有词汇含义，只起连接作用，其作用大致相当于一个冒号；而引导定语从句的that是关系代词，且在句中充当句子的成分(主语或宾语)。如：

I hold the belief that where there is a will, there is a way. (that引导的是同位语从句，它在从句中不充当句子成分)

The letter that I received yesterday was from my sister. (that引导的是定语从句，它在从句中充当宾语)

二、从意义上来看

同位语从句所表达的内容与作为中心词的名词所表达的内容是一样的，两者之间可以划等号；而定语从句则是限制或修饰中心词的，中心词所表达的内容不等于定语从句所表达的内容。如：

He told me the news that his uncle would come home from abroad on National Day. (that引导的是同位语从句，其中the news = his uncle would come home from abroad on National Day)

This is the factory that we visited last month. (that引导的是定语从句，其中 the factory≠ we visited last month.)

三、从中心名词上来看

同位语从句中的中心名词常是一些表示概括意义的抽象名词，这类名词有fact, idea, view, thought, order, suggestion, news, truth, hope, promise, answer, belief, condition, doubt, fear, problem, proof, question 等。而定语从句中的中心名词是没有限定的。如：

四、从that是否可以省略来看

引导同位语从句的连词that一般不能省略，而在定语从句中，that作宾语时，可以省略。另外，引导定语从句的that若指事物，它可以用which来代替，而在同位语从句中，that 永远不能用which替代。