1. 从词类上区别

　同位语从句前面的名词只能是idea,fact,news,hope,belief,suggestion,proposal,word,thought,doubt,truth,possibility,promise,order等有一定内涵的名词?而定语从句的先行词可以是名词?代词?主句的一部分或是整个主句?如:The possibility that the majority of the labour force will work at home is often discussed.人们经常讨论大多数劳动力将会在家里工作的可能性?(同位语从句)

We are not looking into the question whether he is worth trusting. 我们正在调查的问题不是他是否值得信赖的问题?(同位语从句)

Word came that he had been abroad. 据说他已经出国了?(同位语从句)

Our team has won the game, which made us very happy. 我们的队赢了,这让我们很高兴?(定语从句)

The doctor whom you are looking for is in the room. 你找的那位医生在房间里面?(定语从句)

His mother did all she could to help him with his study. 他妈妈尽她的最大努力帮助他的学习?(定语从句,代词all作先行词?)

　2. 从性质上区别

　定语从句是从句对其先行词的修饰或限制,属于形容词性从句的范畴;而同位语从句是从句对前面抽象名词的进一步的说明和解释,属于名词性从句的范畴?如:

The news that our team has won the game was true. 我们队赢了那场比赛的消息是真的?(同位语从句,补充说明news到底是一个什么消息?)

The news that he told me yesterday was true. 昨天他告诉我的那个消息是真的?(定语从句,news在从句中作told的宾语?)

I made a promise that if anyone set me free I would make him very rich. 我许诺如果谁让我自由,我就让他非常富有?(同位语从句,补充说明promise到底是一个什么诺言?)

The mother made a promise that pleased all her children. 妈妈做出了一个令她的孩子们高兴的许诺 (定语从句,promise在从句中作pleased的主语)

　3. 从引导词及其在句子中的成分上区别

　有些引导词如how, whether, what可以引导同位语从句,但不能引导定语从句?如:

That question whether we need it has not been considered. 我们是否需要它这个问题还没有考虑?(同位语从句)

I have no idea what has happened to him. 我不知道他发生了什么事 (同位语从句)

　引导词that引导定语从句时,在从句中一般作主语或宾语(指物时还可以用which代替),并且作宾语时常常省略?that在同位语从句中仅起连接作用,不充当任何成分,并且不能省略,也不能用which来代替?如:

The order that we should send a few people to help the other groups was received yesterday. 我们应派几个人去帮别的几个小组的命令昨天收到了 (同位语从句,是对order的具体解释,that虽不作成分,但不能省略)

The order that we received yesterday was that we 焥hould send a few people to help the other groups. 我们昨天收到的命令是我们应该派几个人去帮助别的几个小组?(定语从句,是名词order的修饰语,that在从句中作received的宾语,可以省略