A
关系代词that
1）不用that的情况
a） 引导非限定性定语从句时.如：
（错）The tree, that is four hundred years old, is very famous here.
b） 介词后不能用.如：
We depend on the land from which we get our food.
We depend on the land that/which we get our food from.
2） 只能用that作为定语从句的关系代词的情况
a） 在there be 句型中,只用that,不用which.
b） 在不定代词,如：anything, nothing, the one, all, much, few, any, little等作先行词时,只用that,不用which.
c） 先行词有the only, the very修饰时,只用that.
d） 先行词为序数词、数词、形容词最高级时,只用that..
e） 先行词既有人,又有物时.如：
All that is needed is a supply of oil.
Finally, the thief handed everything that he had stolen to the police. .
（3）．只用which is 情况有两种情况：一是非限制性定语从句,关连词要用which；二是做介词宾语时只能用which.请看下列两组例句：
I said nothing , which made him more angry .
I have the book about which you are talking .
关系代词as与which的区别
在定语从句的非限制性定语中,which 和as 是容易混淆的.在很多情况下两者兼可互换,但在有些情况中又不可互换：
1. 当先行词不是一个单词,而是整个主句时,一般既可以用which,又可用as．例：He was a foreigner,as(which)I knew from his accent．
2. 当主句是从句的原因时,多用which.
例：He changed his mind,which made me very angry． (which代替he changed his mind在定语从句中作主语,也是使我生气的原因.)
