I know the reason why he came late.

The reason why he was late was that he was ill.

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非限定性定语从句

1.　　 who 指人，做主语。

Yesterday I met Li Ping, who seemed to be very busy.

做定语从句的地点状语。

This is the place where we lived for five years.

The factory where his father works is in the west of the city.

The factory in which his father works is in the west of the city.

( 介词+which可以代替where)

[ This is a place which//that I wanted to visit. ]

做定语从句的时间状语。

I still remember the day when I first came to Beijing.

I still remember the day on which I first came to Beijing.

( 介词+which可以代替 when)

I’ll never forget the time when we worked on the farm.

[I remember the days which//that we spent together.]

as用作关系代词和关系副词引导限定性定语从句，在从句中作主语、定语、表语、或状语，常与the same … as; such… as , so… as , as… as 连用。

I like the same book as you do.

I want to have such a dictionary as he has.

I shall do it in the same way as you did.

He is the same age as you ( are ).

He will give you such information as will help you.

Such students as you have mentioned should be criticized.

=Students such as you have mentioned should be criticized.

在the same…后也可用that引导定语从句，但意义有所不同：

that 引出的定语从句，指的是与先行词同一事物；而 as引出的定语从句是与先行词相似的同类事物。

I want to use the same tool that I used yesterday.

I want to use the same tool as I used yesterday.

①.　 先行词为不定冠词all, few, little, much, everything, nothing 等.

All (that) you have to do is to practise every day.

There isn’t much (that) I can do.

②.　 先行词被序数词或形容词的最高级所修饰时.

The first lesson (that) I learned will never be forgotten.

This is the best film (that ) I have ever seen.

③.　 先行词被all, any, every, few, little, no, some 等不定代词修饰时.

I have read all the books (that) you gave me.

④.　 先行词被the only, the very, the last, the same 等修饰时.

The white flower is the only one that I really like.

This is the very book that I want to read.

This is the same book that I want to read. (同一本书)

----This is the same book as I want to read.

⑤.　 当主句以who或 which 开头的特殊疑问句时,定语从句that 引导.

Who is the man that is standing there ?

Which of us that knows something about physics doesn’t know this ?

⑥先行词既有人又有物时。

We talked about the persons and things that we could remember.

⑦time做先行词，前面如果有序数词或 last 来修饰时，定语从句用 that 来引导或省略；如果没有序数词或 last 来修饰时，既可用 when　 也可用that来引导。

This is the last time ( that ) I shall give you a lesson.

The first time ( that ) I saw him was in 1972.

It’s time ( that ) we got up.

修饰表示物的先行词. 先行词可以是词、短语、句子。

This is the book (which/that ) you want.

The building which//that stands near the river is our school.

The room in which there is a machine is a workshop.(介词后面的which不能省略)

The house (which/that) we live in is not large.

This is the watch (which/that ) he was looking for.

(在含有介词的动词固定词组中,介词只能放在动词之后,不能放在which之前 )

作宾语时可省略,不能放在介词后面作宾语.

The letter (that//which) I received was from my father.

I can’t find the letter that//which came this morning.

Do you still remember the days (that//which)we spent together ?

This is the man that//who helped me.

The house (that//which) we live in is not large.

=The house in which we live is not large.

The doctor (that//whom //who)you are looking for is in the room.

Do you know the woman whose name is Mary.

Pass me the dictionary whose cover is black.

口语中可以省略.

The doctor (whom) you are looking for is in the room.

The person (whom) you just talked to is Mr. Li.

=The person to whom you just talked is Mr. Li. (介词后 whom不能省略)

The babies whom the nurses are looking for are very healthy.

(固定短语中的介词一般不省略,介词仍放在动词之后)

修饰表示人的先行词. 例如:

This is the man who helped me.

Do you know the comrade who spoke just now ?

The boy who//whom we saw yesterday is John’s brother.