非限制性定语从句which和as的用法
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.
A.It B.As C.That D.Which
答案：B
_______ is mentioned above,the number of the students in senior high schools is increasing.
A.Which B.As C.That D.It
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 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。
只用as而不用which：
一、当定语从句置于主句前面时：
例：As you see, the Chinese people are hard-working. (定语从句在句首)
你知道，中国人民是勤劳的。
注意下面句子的多种表达方法：
众所周知，月球每月绕地球转一圈。
1.As is known to everybody, the moon travels round the earth once every month.
2. The moon travels round the earth once every month, as/which is known to everybody.
3. It is known to everybody that the moon travels round the earth once every month.
4.What is known to everybody is that the earth travels round the earth once every month.
(后两句属名词性从句范畴)
二、先行词做主语且定语从句为被动语态时：
此时，从句谓语通常为： be known, be said, be reported， be announced, be mentioned等。如果从句是主动语态，一般用which作主语。
例1：She has been absent again, as is expected.
她又缺席了，这在预料之中。
例2：Tom has made rapid progress, which makes me very happy.
汤姆进步很快，这使我很高兴。
另外，as常用在下列习惯用语中：as (it) seems likely, as (it) often happens, as (it) was printed out, as (it) was said earlier, as I remember (it), as I understand (it), as (it) appears, as it often the case, as anybody can see, as we have expected.
例1：Jack has won the first prize, as it often happens.
像往常一样，杰克得了一等奖。
例2：She has read widely in Romantic Literature, as it appearsfromher essay ·
她广泛涉猎了浪漫主义文学，这从她的文章中可以看出来。
只用which而不用as
一、关系代词代替前面主句中的宾语从句或定语从句的谓语动词后有复合宾语时：
例：I don't think that he will come to see me, which makes me sad.
我认为他不会来看我了，这使我伤心。
二、which一般用作实义动词的主语，这时它所引导的从句与主句之间存在着逻辑上的因果关系时：
例：Tom was late for school again and again, which made his teacher very angry.
汤姆一次又一次上学迟到，这使得他的老师很生气。
三、当非限制性定语从句是否定句时：
例：He pretended not to know me, which I didn't understand.
他假装不认识我，我真不明白。
四、非限制性定语从句中的be动词不能省略时，用which反之用as.
例1：Jane told me she won the match, which was a lie.
简告诉我她赢了这场比赛，这是谎话。(was不可省略)
例2：As (was) planned, we met at the airport.
按照计划，我们在机场相见了。(was可省略)
