同位语从句所修饰的名词比较有限,通常有hope, wish, idea, news, fact, promise, opinion, suggestion, truth等,而定语从句所修饰的名词则非常广泛。
另外,when和where 引导定语从句时,通常只修饰表示时间和地点的名词,而它们引导同位语从句时却不一定;又如why引导定语从句,它通常只修饰名词the reason,而它引导同位语从句时则不一定。
I have no idea when they will come.
I’ll never forget the days when I lived there.
We don’t understand the problem why this is the best choice. 我们不明白这个问题,为什么这是最好的选择。(同位语从句 )
The reason why he didn’t come to the meeting is that he is ill. 他未能来开会,原因是他生病了。(定语从句)