一、從句法功能上來看
同位語從句中連接詞that在從句中不作句子成分,也沒有詞匯含義,只起連接作用,其作用大致相當于一個冒號;而引導定語從句的that是關系代詞,且在句中充當句子的成分(主語或賓語)。如:
I hold the belief that where there is a will, there is a way. (that引導的是同位語從句,它在從句中不充當句子成分)
The letter that I received yesterday was from my sister. (that引導的是定語從句,它在從句中充當賓語)