非限制性定语从句的关系词不能省略，没有关系词的非限制性定语就不是定语从句 而是分词短语作非限制性定语。 如,He passed all the exams this term,which made his parents very happy. 也可以这样表达,He passed all the exams,making his parents very happy. 前者是非限制性定语从句，后者是现在分词短语作非限制性定语（也可以看作结果状语）
限制性定语从句即与主句关系密切，不能分开。去掉使得句意表达不完整。连词主要有 that、which、who、where 等，位置并不固定，位置主要看句中定语从句所修饰的句子成分。 例： the girl who is reading under ther tree is my sister.（修饰主语） i bought a interesting book which was borrowed by my friend yesterday.（修饰宾语）