我们知道，as和which作为关系代词都可以用来引导非限制性定语从句，指代整个主句所表达的内容，在从句中作主语或宾语，但它们在用法上有区别，这一点许多学生往往搞不清楚。现就它们引导非限制性定语从句的用法简述如下：
一、相同之处
当从句位于主句之后，引导词指代整个主句所表达的全部意义时，as和which可以互换。如：
The elephant is like a snake,as/which everybody can see.
大家都看得出，这头大象像条蛇。
A big earthquake occurred there last night,as/which was reported on TV.
据电视报道，昨夜那里发生了大地震。
She is from Shanghai,as/which I know from her accent.
她来自上海，那是我从她的口音知道的。
二、不同之处
1、as引导非限制性定语从句时，位置比较灵活，可位于主句之前，也可位于主句之中或之后;而which引导非限制定语从句只能位于主句之后，不能位于主句之前。如：
_______ is known to everybody,the moon travels round the earth.
h
答案：B
_______ is mentioned above,the number of the students in senior high schools is increasing.
h
答案：B
Air,as we know,is a gas.众所周知，空气是一种气体。
He is very careful,as his work shows.
他的工作表现表明他非常谨慎。
as引导非限制性定语从句常有“如同......那样”的含义，因此在一些固定结构中，如：
as we know;as is known to all(众所周知)
as we all can see(正如我们大家都能看到的那样)
as has been said before/above(正如前面所述)
as has been pointed out(正如所指出的那样)
as might be imagined(可以想象得到)
as might be expected(正如所预料的那样)
as is often the case(情况通常是这样)
以上种种固定搭配，一般不能用which代替as。
2、搭配上，在由the same或such所修饰的名词后通常用as作关系代词引出非限制性定语从句。如：
I have got into the same trouble as he (has).
我遇到了与他一样的麻烦。
I've never heard of such a moving story as he is telling.
我从未听过像他讲得那么有趣的故事。
3、which引导的从句在意义上相当于一个并列句，可以用and this代替，意思是“这件事”;as引导的从句主要起连接上下文的作用，表达说话人的观点、看法，并指出主句内容的根据或出处等。如：
He sold his new car,which surprised me.
=He sold his new car,and this serprised me.
Einstein,as we know,is a famous scientist.
我们都知道，爱因斯坦是一位著名的科学家。
As is known to all,Taiwan is part of China.
众所周知，台湾是中国的一部分。
4、当which在从句中指代的是先行词本身而不是主句时，不可用as代替which。如：
The peach tree,which I planted last year,has not borne any fruit.
这棵桃树是我去年种下的.，还没有结出任何果实。
The book,which I bought yesterday,is very instructive.
我昨天买的这本书很有教育意义。
5、在从句中作主语时，which既可作系动词be的主语，也可作实义动词的主语，而as只可作系动词be的主语。如：
He married her,which was nature.(可用as代替which)
他和她结婚了，这是很自然的事。
She met the boy,which delighted her.(不可用as代替which)
她见到了那个男孩，这使得她很高兴。
He failed in the exam again,which made his parents very angry.
他考试有不及格了，这是他父母很生气。
6、as引导的从句在意义上不可与主语相悖，而which引导的从句则不受此限制。
Mum treats me like a baby,which I can't bear.
妈妈把我当小孩看待，这是我不能忍受的。
