連接詞作用不同。
以that為例，連接定語從句的that一定會在從句中充當一定的成分;而連接同位語從句的that只起連接作用，不在從句中擔任任何句子成分。whether 和 how 可以引導同位語從句，但不能引導定語從句。關系代詞和關系副詞除了連接從句的作用外，還在定語從句中充當一定的句子成分，如主語、賓語、定語、表語等。引導定語從句的連接詞是關系詞，常見的關系代詞和關系副詞有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)先行詞不同。
定語從句先行詞既可以是人，也可以是物。相當于形容詞。
The moon is a satellite that goes? round the earth.
The few points that the president stressed in his report are very important.
同位語從句進一步說明的是具有抽象概念的詞，其作用相е當于名詞，對前面的名詞進行補充說明。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 , questiоon , 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)從引導詞來看
如果引導從句的詞為when表示時間，where表示地點，why表示原因，how怎么樣，表示方式，而它前面的詞分別表示時間，地點，原因，方式等意義，那它們就是關系副詞，引導的是定語從句。
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.
