一、
定语从句对前面的名词具有修饰、限制或描述功能；
同位语从句对前面的名词具有补充、解释、说明功能.如：
The fact that she told us is believable.她对我们说的话是可信的.(that引导定语从句)
The fact that he once joined the army is believable.他曾当过兵这一事实是可信的.(that引导同位语从句)
二、that在引导定语从句时,不光要起连接作用,还要在从句中作成分,若that在从句中作宾语,则that可以省略；而that引导同位语从句时,只起连接作用,不充当从句中的任何成分,且一般不能省略.如：
Don’t you forget the promise (that) you made?难道你忘了你许下的诺言吗?(that在定语从句中作宾语,可省略
)He makes a promise that he will make you very rich if you work with him.他许下诺言：如果你和他一起干,他会使你很富有.(that在同位语从句中不作任何成分,且不省略)
三、定语从句前面的名词既可以是具体名词,也可以是抽象名词；而同位语从句所修饰词一般是少数抽象名词,从句说明该名词所指的具体内容,如：advice,answer,belief,doubt,fact,hope,idea,information,message,news,order,promise,proposal,possibility,story,suggestion,thought,truth,view,word等.
The patient accepted the advice that the doctor gave him.病人接受了医生给他的建议.(定语从句限定advice)
The patient accepted the advice that smoking should be given up.病人接受戒烟这一忠告.(同位语从句所表述的是advice的具体内容)
This is the book that I bought yesterday.这是我昨天买的那本书.(
先行词为具体名词)