在非限定定语从句中as 和which 都能做关系代词，指代主句所表示的整个情况或其中的某一部分，有时可以换用。如：

She is found of sports,as/which we are glad to hear.

She won the first place in the match,as/which was expected.

He failed in the history examination,as/which seemed natural.

但as 和which 在用法上还有区别。如：

As we all know,the earth goes round the sun .(as 不等于which)

He has to work on Sundays,which he doesn't like.(as 不等于 which)

The building over there,which has a red star at it's top,is our new factory.

(as 不等于which)

as和 which的主要异同总结为以下几个方面：

1.位置的不同

   which 引导的非限定定语从句常放在主句之后；as 引导的 非限定定语从句常放在句首，句中，句末。如：

As is well-known,this kind of computer is out of date.(as 不等于which)

He was a foreigner,asI know  from his accent.(as 等于which)

2.可换用的情况

（１）关系代词在非限制性定语从句中做宾语。如：

     This elephant is like a snake,as/which anybody can see.

     He found the book,as/which was what we had wanted.

 （２）关系代词在非限制性定语从句中做主语，其谓语动词为be动词；不及物动词或及物动词的被动语态。如：

     She has been late again,as/which was expected.

     The team won the game ,as/which was what we had wanted.

3. 不可换用的情况

 （１）非限定性定语从句与主句语意一致，即非限制性定语 从句在意义上不与主句相抵触常对主句意义起一定的辅 助作用，用as;反之用which。如：

    She has married again,as we expected.

    The elephant is like a snake,which is not right.

  (２）在从句为主谓宾结构中，关系代词做主语时只用  which。如：

     He made rapid progress,which delighted his father. （主语）

   （３）在从句为主谓宾补结构中，关系代词做主语或宾语时，只用which.

    如：

        He started telling lies,which she felt strange. （宾语）

        His dog became ill and died,which made him very sad.（主语）

    （４）as常用的短语

       as often happens;as was pointed out;as was said earlier;as I  remebered;as we all know;as was known;the same as;such as....

      如：

         Tom has passed his examinations,as often happens.

          She has reads widely ,as appears from his articles.

      (5)用as 或which填空

  1.She was married to that man,_____ was not expected.

  2.John got beaten in the game,____ had been expected.

  3._____ has been said above ,grammar is a set of rules.

  4.Cars,___ anyone can see ,run faster than tractors.

  5.We have to work in the evenings,____we don't like.

  6.He said that he had never seen me before ,___ was not true .

  7.There is great disorder,___ the newspaper will have told yes.

  8.I never saw such bad work ,____ Jack had done.

  9.This is the same story ___ my brother told me.

  10.My parents treat me as a child,___ I can't bear.

  11.Betty always tells lies,___ her parents find strange.

  12.He opposed the idea,___ could be expected.

  13.Mr Smith usually praises his student Rose in public,__ she  doesn't like at all.

  14.He saw the girl,____he hoped he would.

  15.Light travels in straight line,___ is a well known fact.

keys: 1.which   2.as   3.As    4.as    5.which    6.which   7.which/as    8.as    9.as     10.which    11.which   12.as    13.which        14.which/as     15.which/as