连接词作用不同。

以that为例，连接定语从句的that一定会在从句中充当一定的成分;而连接同位语从句的that只起连接作用，不在从句中担任任何句子成分。 whether 和 how 可以引导同位语从句，但不能引导定语从句。关系代词和关系副词除了连接从句的作用外，还在定语从句中充当一定的句子成分，如主语、宾语、定语、表语等。引导定语从句的连接词是关系词，常见的关系代词和关系副词有that , which , who , whose , whom , when , where , why , as，than, but等

The fact that we talked about is very important.

The fact that he succeeded in the experiment pleased everybody.

(二)先行词不同。

定语从句先行词既可以是人，也可以是物。相当于形容词。

The moon is a satellite that goes round the earth.

The few points that the president stressed in his report are very important.

同位语从句进一步说明的是具有抽象概念的词，其作用相当于名词，对前面的名词进行补充说明。如fact, result, discovery，belief , doubt , fact , hope , idea , news , possibility , thought , order , suggestion , wish , answer , information , conclusion , decision , discovery , knowledge , law , opinion , problem , promise , proof , question , report , truth , risk等等。

He has an idea that he can make the scientific instrument in a better way.

He expressed the hope that he would come to China.

(三)从引导词来看

如果引导从句的词为when表示时间，where表示地点，why表示原因，how怎么样，表示方式，而它前面的词分别表示时间，地点，原因，方式等意义，那它们就是关系副词，引导的是定语从句。

I’ll never forget the day when I saw the Great Wall.

This is the room where LuXun once lived.

The question when we will start the work is not decided.

You have no idea how worried I was.