作賓語時可省略，不能放在介詞後面作賓語。
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.
as用作關係代詞和關係副詞引導限定性定語從句，在從句中作主語、定語、表語、或狀語，常與the same … as; such… as ， so… as ， as… as 連用。
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…後也可用that引導定語從句，但意義有所不同：
that 引出的定語從句，指的是與先行詞同一事物；而 as引出的定語從句是與先行詞相似的'同類事物。
I want to use the same tool that I used yesterday.
I want to use the same tool as I used yesterday.
Do you know the woman whose name is Mary.
Pass me the dictionary whose cover is black.
①。 先行詞為不定冠詞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。 先行詞被序數詞或形容詞的最高級所修飾時。
The first lesson (that) I learned will never be forgotten.
This is the best film （that ） I have ever seen.
③。 先行詞被all, any, every, few, little, no, some 等不定代詞修飾時。
I have read all the books (that) you gave me.
④。 先行詞被the only, the very, the last, the same 等修飾時。
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. （同一本書）
----This is the same book as I want to read.
⑤。 當主句以who或 which 開頭的特殊疑問句時，定語從句that 引導。
Who is the man that is standing there ？
Which of us that knows something about physics doesn’t know this ？
⑥先行詞既有人又有物時。
We talked about the persons and things that we could remember.
⑦time做先行詞，前面如果有序數詞或 last 來修飾時，定語從句用 that 來引導或省略；如果沒有序數詞或 last 來修飾時，既可用 when 也可用that來引導。
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.
做定語從句的時間狀語。
I still remember the day when I first came to Beijing.
I still remember the day on which I first came to Beijing.
（ 介詞＋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.]
口語中可以省略。
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. （介詞後 whom不能省略）
The babies whom the nurses are looking for are very healthy.
（固定短語中的介詞一般不省略，介詞仍放在動詞之後）
修飾表示人的先行詞。 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.
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