which
as
1、语义的区别
当主句和从句语义一致时,用as；
He made along speech,as we expected.
而且 定语从句中先行词（place）被such修饰时,关系代词一般用as
当主句和从句语义转折不一致时,用which,
He made along speech,which was unexpected.
2、位置的区别
看到句首一定用as,which 不可能放在句首.
as we all know,
as is mentioned above
as often happens
as is often the case
As is well known
3、含义的区别
as有“正如”的含义,which没有此含义.
As is well known,Shanghai is a big city．
Some of the roads were flooded,which made our journey more difficult．
4、否定句用which
Tom drinks a lot everyday,which his wife doesn’t like at all.