什么情况下“介词+which” 代替先行词

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问：罗教授，你好！请教一个问题：定语从句中当先行词做状语不可省略有时可以用“介词+which”代替。一般在什么情况下“介词+which” 代替先行词。

  答：我们看看几个不同形式的定语从句，同时作些针对性的解释：

1）    This is the book which you want to read. (这是你要阅读的书。---- 关系代词which作定语从句to read的宾语。在这种情况下，可以省去关系代词：This is the book you want to read.)

2）    This is the book about which you have talked to me. (这是你曾经给我谈起的那本书。---- 关系代词which在定语从句中作介词about的宾语，which的先行词只是前面的the book。当引导定语从句的关系代词前面有介词时，只能用关系代词which。在口语中，一般喜欢把介词放到定语从句的后面去, 同时省略掉关系代词which：This is the book you have talked about to me. 这就是你所问的那种情况。)

3）    This is the book on the cover of which there is a map of China. (这是封面上有张中国地图的那本书。---- 关系代词前面不是一个介词，而是一个介词词组，这时也只能用关系代词which，不能用that。和上面例句不同的是：不能将这个介词词组放到定语从句的后面去，我们一般不能说：This is the book there is a map of China on the cover of .)