which非限制性定语从句
■关系代词which只用于指事或物，不用于指人，它在定语从句中主要用作主语或宾语。如：
She was not on the train which arrived just now. 她不在刚才到达的那列火车上。(关系代词which指train，在从句中用作主语)
He never got back the money which he had lent him. 他从未收回他曾借给他的那笔钱。(关系代词which指money，在从句中用作宾语)
■which用于引导限制性定语从句外，还可以引导非限制定语从句。如：
The ship struck an iceberg, which tore a huge hole in her side. 船碰在一座冰山上，船帮被撞出一个大洞。
She gave me this jumper, which she had knitted herself. 她送给我这件羊毛衫，这是她亲手织的。
This machine, which I have looked after for twenty years, is still working perfectly. 这台机器我已经照管了20年，现在仍然没一点儿毛病。
■which有时也可用作定语，尤其用于“介词+which+名词”这类结构中，此时的which含有this或that的意味。如：
He may be busy, in which case I’ll call later. 他可能忙，如若如此，我以后再来拜访。
I may have to work late, in which case I’ll telephone. 我可能得晚点下班，那样我会打电话的。
He lost his temper, at which point I decided to go home. 他发脾气了，这时我就决定回家了。
He was appointed Lord Chancellor, in which post he spent the rest of his life. 他被任命为大法官，在这个职位上度过了他的后半生。
The postman comes at 6.30 in the morning, at which time I am usually fast asleep. 邮递员早晨6点30分来，这个时候我通常还睡着大觉呢。
It was derived from Posidonius, for which reason much of its information may well have been out of date. 它源于鲍森杜尼尔斯。由于这个原因，它的大部分信息很可能已经过时。
有时其前也可以没有介词。如：
He is studying economics, which knoweledge is very important today. 他学经济学，这种知识现今很重要。
He spoke in Greek, which language I could only follow with difficulty. 他说希腊语，我听起来很吃力。
■ 关系代词which有时可用于指人。原则上说，which用于指事物，不用于指人。但以下情况属于例外：
1. 用于在指人的名词之后，指不强调性别的婴儿：
He was the last child which was born. 他是最后出生的孩子。(which指小孩)
2. 当不是指具体的某人，而是指人的属性（如职业、身份、地位、职务、品质、特征等）时，则也用关系代词 which而不用 who。如：
They accused him of being a traitor, which he was. 他们指控他是叛徒，他真是叛徒。
She talked like a native, which she hardly was. 她说起话来像是个本地人，其实她不见得是。
■ 在非常正式的文体中，代词that后可以接一个修饰它的定语从句，这个定语从句通常用which来引导。如：
They say the only real knowledge is that which can be measured. 据说唯一真实的知识是可以检验的知识。
在现代英语中，that which通常用what来代替。如：
We have that which (=what) we need. 我们需要的东西都有了。
That which (=What) many people say is not always true. 很多人说的话并不表示一定是真的。
非限制性定语从句which和as的用法
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.
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 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。
