定语从句是高中课本中的重点和难点，而“介词+关系代词”引导的定语从句尤其复杂.
1.简单介词+关系代词
在这种结构中，关系代词指人时用whom，指物时用which.关系副词when，where，why的含义相当“介词+which”，因此常相互交替使用.When＝on/in/by+which，where＝at/in/from+which，why＝for which.eg：
①There are occasions when（on which）one must yield.
②Beijing is the place where （＝in which ）I was born.
③Is this the reason why （＝for which）he refused our offer？
介词的选择要三方面考虑：一是考虑定语从句中谓语动词的习惯搭配；二是考虑先行词与从句中的动词关系和所表达的含义；三是考虑先行词在定语从句中的作用和含义.eg：
①Do you know the teacher with whom Liping just shook hands？
（with来自于动词短语 shake hands with）
②There are fifty students in our class ，of whom Tom is the most deligent.
（of表示部分和整体的关系）
③He invented a machine ,with which he could harvest rice .（with表工具）
“简单介词+关系代词”结构中介词的位置大致分三种情况：1）介词放在关系代词前 2）介词在从句谓语动词后 3）介词既可放在关系代词前，又可放在从句谓语动词后.
1）介词与先行词构成固定搭配的介词短语时，介词须放在指代先行词的关系代词前.如：
①This is the desk on which I found my book.（关系代词指代先行词the desk ，并且与介词on构成介词短语on the desk, 作从句地点状语）
②Human facial expressions differ from those of animals in the degree to which
they can be controlled on purpose.
（介词to 与先行词degree 构成固定短语to the/some degree ）
③There was a time in history during which the blacks were treated as slaves.
（介词during与先行词a time构成介词短语）
④I don’t like the way in which he treats us. （先行词为表方式的名词the way 且在定语从句中作状语时，关系词还可用that或省略关系词，而不只看固定搭配的介词短语in the way，此情况较特殊.）
2）介词须放在从句谓语动词后.这种情况下介词与定语从句的谓语动词连用构成固定的动词短语.如：
①He is the man (who/whom/that)I’m looking for.
（此句中定语从句的谓语动词是固定短语look for，介词for与look不能分开，因此介词for不能提到关系代词前）.
②He is the child whom my grandma has been taking care of .
③This is the letter which I’m looking forward to.
以上②③中固定动词短语take care of，look forward to 的介词of，to同样不可提到关系代词前）.
3）当介词可与先行词构成短语搭配，也可与定语从句的谓语动词连用构成动词短语时，介词位置可放在关系代词前，也可放在从句谓语动词后.如：
①Do you know the person(who/whom/that) I talked to just now?=Do you know the person
to whem I talked just now?
（介词to 既可与先行词 the person 构成 to the person ，又可与从句谓语动词talk 构成talk to，因此to的位置可前可后）
②The English play in which my studerts acted at the New year’s party was a great success.
=The English play (which/that) my students acted in at the New year’s party was a great success. （介词in的位置同理有两种）
特别注意：介词位置的不同，会对关系代词产生一些微妙的作用.在“介词+关系代词”结构中，介词后的关系代词只能用which （指物）或whom（指人）.介词若放在关系代词前，关系代词不能省略，若介词出现在从句谓语动词后，则关系代词可以而且常常省略.如上例①②
2.复杂介词+关系代词
在这种结构中，可使用的关系代词有which，whom 和whose，常用的复杂介词有at the back of ，as a result of ，because of ,by means of ,in front of ,in support of ,on account of等.如：
①They arrived at a farmhouse,in front of which sat a boy.
②There were some mistakes in my homework,because of which I couldn’t go home.
“复杂介词+关系代词”结构中介词匀位于关系代词前.
3.表示部分的词+of+关系代词
在这种结构中，指人的关系代词用whom，指物的用which.表部分的词有one，second，some，many，several，a quarter，half等，如：
①We have got 16 medals,most of which were won by women.
②Some students in our class are studying abroad,three of whom are girls.
4.the+名词+of+关系代词
在这种结构中，关系代词只能用which，它表示的是一种所有关系.我们也可以用“whose+名词”和“of which the+名词”来代替.如：
①I have read the novel,the tittle of which (whose tittle /of which the tittle) is Red and Black.我读过名叫《红与黑》的小说.
②I live in the house ,the window of which(whose window/of which the window)faces
south.我住的房子窗户朝南.
5. 简单介词+关系代词+名词
这种结构中关系代词可使用which 和whose .选择介词时要考虑关系代词后的名词以及整个句子的意义.如
①The boss in whose department Mr.king worked had heard atout the accident.
②This is the editor for whose magazine LiMing writes.
③He is a millionare with whose money many houses have been built for the poor.
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