这个问题我已经回答过了,现在给你再抄一遍：
简单地说,如果定语从句和主句分不开,就用that.如果能分开,就用which.通常which的定语从句和主句之间是有逗号的,而that的从句永远不能用逗号和主句分开.
that和which的区别是限制性定语从句和非限制性定语从句的区别.that引导限制性定语从句,通常是不能省略的,如果省略了句子就不通了(或者表达的意思就变了).which引导非限制性从句,如果省略并不影响句子的整体表达.
限制性定语从句的例子：
Their dreams,and their songs,and their stories,are of the road that has no end.
这句话如果你把定语从句that has no end去掉,意思就不对了.
An adjective clause that is restrictive should not be set off by commas.
同样,这句话如果没有定语从句that is restrictive,意思就不对了.
非限制性定语从句的例子：
Thus the next task began of clearing Arab forces from the area and then rebuilding the road,which would be paved and five meters wide.
这句话如果你把定语从句which would be paved and five meters wide去掉,虽然少了从句里的意思,但是不影响主句的表达.
有时侯用限制性定语从句还是用非限制性定语从句完全取决于说话的人想强调什么.比如非限制性：
The painting,which was hanging in the foyer,was stolen.
强调的是画被偷了.而限制性：
The painting that was hanging in the foyer was stolen.
强调的是挂在门厅里的.