定语从句是形容词性的,其功能是修饰先行词,起限定作用.而同位语从句等同于它所修饰的名词,是名词性的,其功能是对所修饰的名词作补充说明.
It is a fact (that) you can't deny. 这是一个你不能否认的事实.(定语从句)
It is a fact that she has done her best. 她尽了最大的努力,这是事实.(同位语从句)
在定语从句中,that代先行词,在从句中不仅起连接作用,还充当某个句子成分,在作宾语时通常可省略.而在同位语从句中,that只起连接主句和从句的作用,无意义,在从句中不充当成分,一般不可省.
The news (that) we heard spread all over the school campus.(定语从句)
The news that Mr. Li will be our new English teacher is true.(同位从句)