在定语从句里,as既可以引导非限定性定语从句,也可以引导限定性定语从句.as在两个里面都是作关系代词.
（1）非限定性定语从句：用法同which,用来指代前面的句子（而非先行词）,多与know/expect/see搭配使用.
e.g.As we know,smoking does harm to our health.
（2）as引导限定性定语从句,多与the same/such/so搭配使用.
e.g.I have got into the same trouble as he has.
另：which和as的区别
A.which不能放句首（The sun heats the earth,which is very important to us.)
B.as如果在从句中作主语,后面的谓语动词必须是系动词.(It rained hard yesterday,which prevented me from going to the park.这边就不能用as来代替which,因为prevent是行为动词）