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语义的区别：当主句和从句语义一致时，用as；当主句和从句语义转折不一致时，用which。位置的区别：看到句首一定用as，which不可能放在句首。含义的区别：as有“正如”的含义，which没有此含义，否定句用which。
定语从句基本概念
在复合句中，修饰某一名词或代词的从句叫定语从句。如：
Do you know the man who spoke at the meeting just now?
That is the house where he lived ten years ago.
定语从句所修饰的词叫先行词；定语从句一般用关系代词或关系副词来引导，关系词放在先行词与定语从句之间起连接作用，同时又作从句中的一个成分。
引导定语从句的关系代词有：that, who, whom, whose, which;
关系副词有：when, where, why.
关系词的用法
（一）关系代词的用法：
1． 作主语用who, which和that, 如：
He is the man who/that lives next door.
The train which/that has just left is for Shenzhen.
2． 作宾语用whom, who, which, that, 如：
The man (whom/who/that) we have just seen is a famous writer.
Where is the book (which/that) I bought last week?
注：在非正式文体中，用于指人的关系代词who whom, that 通常可以省略，但在正式文体中通常用whom, 不可省略；用于指物的关系代词which和that 在非正式文体中也通常省略，但在正式文体中一般不省略。