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关于“介词 + which / whom”引导的定语从句
一、基本构成
1.“介词+关系代词”结构引导的定语从句中，关系代词只能用which（指物）或whom(指人) ，即：介词+which/whom。
（1）The man to whom you spoke is a teacher.你和他说话的那个人是个老师。
（2）The city in which she lives is far away.她居住的城市很远。
（3）The little girl is reading a book, in which there are many cartoons.小女孩正在读一本书，里面有许多卡通画。
注意：当介词放在关系代词的前面时，用在介词后的关系代词不能省略。
2.在限制性定语从句中，当介词置于定语从句的谓语动词之后时，可用that/which(指物)；that/who/whom（指人）作介词的宾语.且此处关系代词可以省略，特别是在口语中。如：
（1) The man (who/whom /that )you spoke to is a teacher.
（2）The city（which/that）she lives in is far away.
注：通常介词放在定语从句中动词后，也可以放在关系代词前面，但有些特殊动词短语搭配不能拆分，介词只能放在动词后，如： look for, look after, pay attention to ,take care of , look forward to, listen to等。
This is the pen that / which you are looking for.这是你要找的钢笔。
The patient whom she is looking after is her father.她照顾的病人是她的父亲。
The words that /which we should pay attention to are written on the blackboard.我们应该注意的单词写在黑板上。
There are fifty patients who/whom/that we must take good care of.有五十个病人我们必须好好照顾。
二、关系副词when，where，why可用“介词＋which”来代替。关系副词实际上是介词＋先行词。其中
when=表时间的介词（如：in，at,during等）+which；
Where=表地点的介词（如：in，at,on,under等）+which；
Why=表原因的介词（如：for）+which
(1)当先行词表示时间，定语从句中缺少时间状语时，通常用关系副词when引导，此时也可选用表示时间的介词＋which来代替关系副词when。
I still remember the day when I came here. ( on the day =when) 我还记得我来这里的那天
(2)当先行词表示地点，定语从句中缺少地点状语时，通常用关系副词where引导，此时也可选用表示地点的介词＋which来代替关系副词where。
This is the house where I lived last year. ( in the house= where)这就是我去年住的房子。
(3)当先行词为reason，定语从句中缺少表示原因状语时，通常用关系副词why引导，此时也可以用for which来代替关系副词why。
There are many reasons why people like traveling. (for the reasons =why)人们喜欢旅游有很多原因

只用that不用which的情况：
1、当先行词是all, little, few, much, something, everything, anything, nothing, none等不定代词时。如：
Everything that happened then was like a nightmare.当时发生的一切就像是一场噩梦。
2、当先行词被only, any, few, little, no, all, just, very（恰好的，表示强调）等词修饰时。例如：
The only thing that we could do was to wait.我们能做的只是等待。
3、当先行词是序数词、形容词最高级或先行词被序数词或形容词最高级修饰时。例如：
The first place that we visited was the Great Wall.我们参观的第一个地方是长城。
4、被修饰词为数词时。例如：
Yesterday I caught two fish and put them in a basin of water. Now you can see the two that are still alive. 昨天我捉到了两条鱼，把它们放在一盆水里。你可以看到那两条鱼还活着呢。
5、主句是There be结构，修饰其主语的定语从句宜用that作关系代词修饰物。例句：
There's still a room that is free.还有一个空房间。

祝学习愉快~