18.2 关系副词引导的定语从句
 
  关系副词可代替的先行词是时间、地点或理由的名词,在从句中作状语。
1)when, where, why
  关系副词when, where, why的含义相当于"介词+ which"结构,因此常常和"介词+ which"结构交替使用,例如:
  There are occasions when (on which) one must yield. 任何人都有不得不屈服的时候。
  Beijing is the place where (in which) I was born. 北京是我的出生地。
  Is this the reason why (for which) he refused our offer? 这就是他拒绝我们帮助他的理由吗?

2)that代替关系副词
  that可以用于表示时间、地点、方式、理由的名词后取代when, where, why和"介词+ which"引导的定语从句,在口语中that常被省略,例如:
  His father died the year (that / when / in which) he was born. 他父亲在他出生那年逝世了。
  He is unlikely to find the place (that / where / in which) he lived forty years ago. 他不大可能找到他四十年前居住过的地方。