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2. 先行词是物时that 和which一般可以互换使用，但有时不可换用。
(1) 下列场合一般用that:
a. 先行词是all, much, anything, something, nothing, everything, little, the
one, none等不定代词
We should do all that is useful to the people.
The little that I have seen of his work is satisfactory.
b. 先行词被序数词，形容词最高级以及first, last, any, only, few, much, no,
some, very等词修饰
The first book that I bought in this bookstore is a dictionary.
The last person that I talked with in London is my girlfriend
The best teacher that I have ever met has gone abroad
You can take any seat that is vacant.
c. 先行词有两个：一个指人，另一个指物
I miss all the people and places that we visited last summer
d. 主句是以who或which开头的特殊疑问句。
e. There be 句型中， There is a seat in the corner that is free.
f. 先行词为数词时
Three buildings have been completed, but there are two that are still under construction.
(2) 下列场合不能用that
a. 在非限定性定语从句中，即逗号后的定语从句不用that引导，指人时用who或whom, 指物时用which.
b. 在“介词+ 关系代词”结构中不用that, 指人时用whom, 指物时用which.
This is the book about which we are talking.
c. 先行词为that, those时，关系词用which指物，who指人
What’s that which you have got in your hand?
d. 两个定语从句，一个用了that, 另一个则用which.
Let me show you the novel that I borrowed from the library which was
newly open to us.
e. 关系代词后有插入语时，只用which
Here are some stamps which – I think – you can take away.
3. 当关系词在从句中作宾语时，关系词可以省略。
4. 当先行词指时间，地点，原因(the reason)时，且关系词在从句中作状语，用when,where, why
如果关系词在从句中作主语，表语或宾语，则用that 或 which.
I want to visit the place where my mother was born.
= I want to visit the place in which my mother was born.
I still remember the years when I studied in the middle school.
= I still remember the years in which I studied in the middle school.
I don’t know the reason why he didn’t agree with us.
= I didn’t know the reason for which he didn’t agree with us.
5. 当先行词是the way, 并且关系词在从句中作方式状语时，可以用that, in which引导，也可省略。
当先行词是time, time 当“次数”讲时，用that引导定语从句，且that可以省略;当time表示“一段时间，时间”讲时，定语从句用when或at / during which引导
I don’t like the way (that / in which) he talked to his mother.
I can hardly remember how many times (that) I’ve failed.
I’ll never forget the time when / at which we visited your hometown.
6. 当先行词family, class, team, army, company等被当作单数时，定语从句中用which,被当作复数时，用who或whom.
The family, which is a large and rich one, was poor.
The party, who are all children, have lost their way.
7. 当先行词是动物时，定语从句一般用which
The dog, which he had kept for five years, was killed by someone yesterday.
B.“介词 + which / whom”引导的定语从句
关系代词在定语从句中作介词的宾语时，常常将介词前置于作宾语的关系词前，I love the music that I can dance to. = I love the music to which I can dance.
The man who I talked to just now is my brother.= The man to whom I talked just now is my brother.
介词后面的关系词指人时只用whom, 不能用who或that; 指物时只用which,不能用that.
注意，并非所有情况下介词都可以前移，含有介词的短语动词不可拆开，介词仍需放在动词后面，如：look for, look after, take care of等。
“介词+ which / whom”前还可以用some, any, none, all, both, neither, many, most, each, few等代词，名词或数词等。
His sons, both of whom loved music very much, spent most of their money on CDs.
The house, the roof of which is blue, will be used as a library.
C. 非限定性定语从句
非限定性定语从句和主句的关系不密切，去掉定语从句，句子的意思仍然完整，形式上用逗号把主句和从句分开，使用时注意以下几点：
l 非限定性定语从句不能用that引导
l 非限定性定语从句中的关系词不能省略。
D.as引导的定语从句的用法
1. as可引导限定性定语从句，用于such…as…., so….as…., the same….as… 结构中。
He bought me such a watch as was advertised in the newspaper.
注意：比较 the same… as…. 和 the same… that….
He bought me the same watch as I lost last week.他买了一块和我上星期丢的一样的表。(一样，但不是同一个)
He bought me the same watch that I lost last week. 他把我上星期丢的那块表又买回来了。(同一个)
2. 当非限定性定语从句的先行词不是主句中的某一个词，而是整个主句时，可以用which 或as引导。
在以下情况下用as引导：
a. 从句意思为“正如/正像...一样”
b. 从句位于句首，构成以下结构：
as is well known 众所周知 as often happens 这种情况经常发生
as is often the case 情况经常这样 as is supposed 如所预料的一样
as has been pointed out 如所指出的 as has been said before 如前所说在下面情况经常用which引导：主句和从句表示因果关系。 He lost the game, which made us very disappointed.
非限定性定语从句是否定意义。He gained a big fortune, which meant nothing to him.
E.其他：
1. 分隔定语从句：定语从句与先行词之间被其他成分分隔开来，这种定语从句叫做分隔定语从句，此时须注意辨别从句的先行词。
The days are gone when we suffered so much.
The boss of the company, whose name was Mr. Joe, told the story.
2. 定语从句的谓语与先行词保持人称和数的一致。
注意：先行词前有one of 修饰，定语从句的谓语用复数;(not the only one of… = one of…)先行词前有the (only) one of 修饰时，定语从句的谓语用单数。
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