as,which都可以引导非限制性定语从句,但俩者适用的场合不同,是不能互换通用的.
1.如果非限制性定语从句的先行词是一件物,不是整个句子,那只能用which,不能用as.
My house,which I bought 1n 2008,is worth 1500 thousand yuan by now ,just ten times as much as that 14 yers ago.
2.如果非限制性定语从句的先行词是整个句子,除了下面3所指的特殊情况外,都只能用which,不能用as：It has rained a lot ,which makes the level of the river rise .
3.如果非限制性定语从句的先行词是整个句子,从句动词又与人的经验知识认知有关,又有~正如.一样~的意思时,正如大家那知道的一样~正如大家都能看得出来,正如你懂的~正如我们所预料的一样,那就只能用as不能用which.例：As we know,the earth is round .As everybody can see,Dawei loves Sussan deep in his heart ,but he dare not to say it out .
4.在限制性定语从句中,如果先行词被such,so修饰,只能用as当关系代词.such.as..."so.as."固定搭配：
He is such a handsome boy that many girls love him .
I am so careless a boy that you mustn`t love .
