（1）从结构上来看,限定性从句不能用逗号与先行词分开；而非限定性从句却必须用逗号分开.引导非限定性从句的关系代词不能省略.例如：
The clock,which my great-grandfather bought,is still in good order.这时钟是我曾祖父买的,现在还是走得很准.
（2）两种形容词性从句往往赋予同一个先行词以不同的含义.例如：
My sister who lives in London is a doctor.我住在伦敦的妹妹是医生.
My sister,who lives in London,is a doctor.我的妹妹是一个医生,她住在伦敦.
（3）非限定性从句可以把整个主句当作先行词,而限定性从句没有这种功能.非限定性从句修饰整个主句时,只能用which或as来引导.例如：
I said nothing,which made him still more angry.我一声不吭,这使他更加生气.
He was a Frenchman,as I could tell from his accent.他是个法国人,我从他的口音中可以听出来.
He was drunk,which seemed to make a bad impression on the policeman.他喝醉了,这似乎给警察留下了不好的印象.
