1、从句所修饰词的不同。同位语从句所修饰词通常是少数一些表示抽象意义的名词，如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。定语从句的先行词可以指人、物等。例如：
He has told us a fact that drinking too much does harm to our health.他又告诉我们一个事实，过渡饮酒对健康有害。(同位语从句)
We have no idea that her mother was a professor ten years ago. . 我们不知道她的母亲十年前是个教授。(同位语从句)
This is the book that I bought yesterday. 这是我昨天买的那本书。（定语从句）
I will never forget the day when I joined the Party. 我将永远不会忘记我入党的那一天。（定语从句）
2. 连接词的作用不同。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 news that he wants to get is whether he will be sent to the countryside.他所想得到的消息是他是否将被派往农村。(定语从句，关系代词that作从句get的宾语。)
Do you have a doubt whether she will be dismissed from school? 你怀疑她是否能被学校开除吗？(同位语从句)
I have no idea how his parents were sad on hearing that their son was killed in the earthquake. 我不知道他的父母一听到他在地震中阵亡是多么悲伤！(同位语从句)
3. 从句作用的不同。定语从句具有形容词或副词的特点，对先行词起修饰、限定作用，描述先行词的性质或特征，与先行词之间是所属关系。同位语从句具有名词的特点，对中心词作进一步补充解释，是中心词的具体内容。例如：
The man that you saw just now is my bother. 你刚才看到的那个人是我的弟弟。(定语从句)
The suggestion that the meeting be put off proved right.推迟会议的建议证明是正确得。（同位语从句）
The suggestion that he had made is that the meeting be put off. 他提出的建议推迟会议。（定语从句）