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)
