介词不提前时，关系代词可省略；
介词提前时，关系代词不可省，即介词＋which（指物时介词后的关系代词只能用which）。
The house which/that was destroyed in the earthquake is weak.
The pen （which/that） you found yesterday is mine.
The games （that/which） the young men competed in were difficult.
The games in which the young men competed were difficult.
※注：介词+关系代词即介词+whom/which（先行词指人用whom，指物用which）。
※5.as指人或物，在定语从句中可作主语，宾语，表语或状语，不能省略。主要用于 “the same…as
…；such …as…；so …as…；as…as…；as follows”固定结构中，形式固定此时的引导限定性定语从句。要用as代替who（m）， which, 或that
引导定语从句：
Such people as knew Hill thought he was honest.
Such people as Hill knew thought he was honest.
My hometown is no longer the same as it was.
Here is so big a stone as no one can lift.
The child knows as much as grow-ups （know）。
I’d like to have the same books as are used in your school.
He is not such a person as I expected.
He will marry as pretty a girl as he can find.
※注：which和as可引导非限定性定语从句：可以指代主句中的一部分或整个句子的内容，which和as都可以指代主句中的一部分或整个句子的内容，有时可以互换。因此，当as/which指代前面的整个句子，或前句中的部分内容作定语从句的主语时，谓语用单数。
Our team lost the game, as/which was reported in the newspaper. She was terrified, as/which I could see from her eyes. He married her, as/which was natural.
区别：
①as引导的非限制性定语从句可以放在主句前面、插在主句中间或放在主句末尾；而which引导的非限制性定语从句只能放在主句后面。
He married her, as/which was natural. =As was natural, he married her. Mark Twin is a great writer, which/as is known to all. =As is known to all, Mark Twin is a great writer.
②as引导的从句有“正如”、“正像”之意，而which则无此意。
常用句型有：
as we all know, as is known to all, as everybody can see, as is expected, As is known/ said/ reported/ told/ we all know等。
如：
As we all know, paper was first made in China. To shut your eyes to facts, as many of you do, is foolish.
③主句和从句有因果关系时，用which. Our class has won the football match, which made us very happy.
Bamboos are hollow, which makes them very light.
④从句含否定意义时常用which.
She didn’t pass the exam, which we couldn’t expect.
She didn’t pass the exam, as we expected.
6.when关系副词：指时间。在定语从句中作时间状语，不能省略。此时的when还可用介词+which替换（此时先行词一定是表时间的名词）。
I still remember the time when I joined the League.
＝I still remember the time on which I joined the League.
＝I still remember the time （which/that） I joined the League on.
7.where关系副词：指地点。在定语从句中作地点状语，不能省略。此时的when还可用介词+which替换（此时先行词一定是表地点的名词）。
I still remember the school where I joined the League.
＝I still remember the school in which I joined the League.
＝I still remember the school （which/that） I joined the League in.
※注：对关系副词when, where的认识。
①。 先行词是时间的名词，并非都用关系副词when引导定语从句。
I’ll never forget the time （which/that） I spent in Beijing.
②。 先行词是地点的名词，并非都用关系副词where引导定语从句。
This is the factory （which/that） he visited yesterday.
③。 当句型为It/This/That is（was） the first（second…last） time引起的句子时用that连接其后的句子。此时的time是次数，不是时间。
It/This is the first time that we travel.
It/This is the last time that I shall give you a lesson.
8. why指原因，在定语从句中作原因状语，先行词通常为reason,不能省略。且why引导的定语从句只能是限定性定语从句。
Why= for which I don’t know the reason why he was late yesterday.
The reason why（for which） he was late is that he missed the bus.
※注：当先行词为reason时，关系副词并非都用why。
This is the reason （that/ which） he gave/ explained to us.
【判断用关系代词还是关系副词】
从句缺少的是主语、宾语还是状语是确定用关系代词还是关系副词的关键。
试比较：
I will never forget the day （which/that） I spent in Hongkong. （ spent是及物动词，后缺宾语，因此用关系代词或省略 ）
I will never forget the day when/on which Hongkong returned to its motherland. （ 从句主谓宾结构完整，缺状语，因此用关系副词或介词加关系代词 ）
The reason （that/which）he gave for not coming to school yesterday isn’t believable. （ gave是及物动词，后缺宾语，因此用关系代词或省略 ）
The reason why he didn’t come to school yesterday isn’t believable. （ 从句主谓宾结构完整，缺状语，因此用关系副词或介词加关系代词 ）
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