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。
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)
We are not talking about the question whether he is worth trusting.
我们现在讨论的不是他是否值得信任的问题。(同位语从句)
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)
3.从引导词上区别
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。
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)
