2．定语从句在选择关连词that 与which时,常常容易出错,所以应记住什么时候只能用that,什么时候只能用which.
（1）．只能用 that 的情况归纳4种：即先得词被序数词最高级修饰过；先行词为all 、everything、nothing等不定代词；先行词同时是人和物并列时；最后先行词分别被the only 、the very the same、the last 修饰.请看下列四组例句,注意其先行词：
①．The first thing that we should do is to work out a plan .
②．I t almost seemed as if the good man were trying to teach us all that he knew at this lesson .
③．Later my father and Mr.Crosset talked for about half an four of things and persons that they remembered in school.
④．This is the very dictionary that I want to find .
（2）．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 .
你的问题涉及到限定性定语从句和非限定性定语从句.
限定性定语：紧跟在先行词后面.
非限定性定语从句：逗号隔开,修饰前面的先行词,或者整句话.具体情况具体分析.
