4、关系词：引导定语从句的都称关系词
关系代词：who, whom, which, that, whose, as。
关系副词：when, where, why。that偶尔也作关系副词。
5、确定关系词的步骤
（1）先找关系词，看先行词指的是什么。
（2）看关系词在从句中所充当的成分。
6、在定语从句中，当先行词指物时，下列情况的关系词宜用that而不用which
（1）先行词被①形容词最高级 ②序数词 ③数词几种词修饰 ④被only、any、few、little、no、all、one of等修饰时。
（2）先行词为all、much、little、none、few、one、something、anything等不定代词时。
（3）先行词中既有人又有物时。
He was looking pleasantly at the children and parcels that filled his bus.
（4）先行词在主句中作表语关系词在从句中作表语时。
The village is no longer the one that was 5 years ago.
（5）当主句中含有疑问词which时。
Which are the books that you bought for me ?
7、宜用which而不用that的情况
（1）在非限制性定语从句中
（2）在关系词前有介词时
（3）当先行词本身是that时
（4）当关系词离先行词较远时