从两个角度讲清that引导定语从句和同位语从句的区别
一、that是否作成分
that在定语从句中充当成分，而在同位语从句中不作任何成分，仅起连接作用。
The rumor that he spread everywhere turned out to be untrue.
The rumor that Tom was a thief turned out to be untrue.
以上两句先行词都是the rumor，引导词都是that。但a句中的that在从句he spread everywhere中作spread的宾语。故a句中that引导的是定语从句。b句中的that在从句Tom was a thief中不作任何成分，故b句中that引导的是同位语从句，作the rumor的同位语，补充说明the rumor的具体内容。
The fact that we talked about is very important. （that作 talk about的宾语，定语从句）
The fact that he succeeded in the experiment pleased everybody. （that不充当成分，同位语从句）
二、从句的作用
定语从句相当于形容词，它对先行词起修饰、描述、限制的作用；同位语从句相当于一个名词，它是前面名词的内容的具体表述，与前面名词是同位关系。
The news that you heard is not true. （定语从句）
The news that Lincoln was murdered at a theatre is true. （同位语从句）
The fact which you pointed out made me interested. （定语从句）
The fact that there are no lives on the moon is known to us all. （同位语从句）
