1）介词后面的关系代词不能省略。
2）that前不能有介词。
3） 某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when ,where和why 互换。
This is the house in which I lived two years ago.
This is the house where I lived two years ago.
Do you remember the day on which you joined our club?
Do you remember the day when you joined our club?
This is the reason why he came late.
This is the reason for which he came late.
介词+关系代词引导的定语从句关系代词在定语从句中做介词宾语时
从句常由介词+关系代词引导
当介词放在关系代词前面时，关系代词只能用which/whom即“介词+which/whom"且不能省略。但当介词位于末尾时可用that/which/who/whom.作介词的宾语，且可以省略。例如：
(1) The school (that/which) he once studied in is very famous."that/which"可以省略
= The school in which he once studied is very famous."which”不可省略
(2) Tomorrow I will bring here a magazine (that/which) you asked for.
= Tomorrow I will bring here a magazine for which you asked.
(3) We’ll go to hear the famous singer (whom/that/who) we have often talked about.
= We’ll go to hear the famous singer about whom we have often talked.
注意：
1. 含有介词的动词短语一般不拆开使用，如：look for, look after, take care of等
T This is the watch which/that I am looking for. (T=正确)
F This is the watch for which I am looking. (F=错误)
2. 若介词放在关系代词前，关系代词指人时用whom，不可用who或者that；指物时用which，不能用that；关系代词是所有格时用whose
(1) The man with whom you talked is my friend. (T)
The man with that you talked is my friend. (F)
(2) The plane in which we flew to Canada is very comfortable. (T)
The plane in that we flew in to Canada is very comfortable. (F)
3. “介词+关系代词”前可有some, any, none, both, all, neither, most, each, few等代词、数词或者名词。
(1) He loved his parents deeply, both of whom are very kind to him.
(2) In the basket there are quite many apples, some of which have gone bad.
(3) There are forty students in our class in all, most of whom are from big cities.
(4) The boat, the name of which is Topsail, is famous.
(5) I bought many books yesterday, three of which are written by Lu Xun.