（1）一般说来，先行词是all, anything, everything, nothing, much, little,
few等，关系代词应用that。如：
That’s all (that) I know.
Everything that can be done is done.
（2）先行词之前有all, any, every, no, little, much, only, very等修饰时，关系代词应用that。如：
He’ll read all the books that are sold here.
Ask any questions (that) you don’t understand.
（3）在以疑问词who开始的疑问句中，避免两个who重复，用that。如：
Who is the man that is shouting there?
正在那边叫喊的那个人是谁？
（4）当先行词是to be的表语，或关系代词本身是从句的表语时，关系代词应用that。如：
It’s a song that is very popular.
它是一首很流行的歌。
My hometown is no longer the place that it used to be.
我的家乡已不是过去那样的地方了。
She is not the girl that she was three years ago.
她已不是三年前的那个女孩了。
（5）先行词被形容词最高级、only等修饰时，应用that。如；
Tang Guoqiang has become one of the best actors that appear on
Chinese screens.
唐国强已成为中国荧屏上最好的演员之一。
（6）先行词人和物两者都有时，要用that。如；
He talked about the teachers and schools that he had visited.
他谈起他拜访过的老师和学校。
（7）当主句以There be开头时，关系代词应用that。如：
There is a seat in the corner that is still free.
（8）关系代词与the same(指同一人/物)连用时，只能用that。如：
This is the same person that I met yesterday.
这就是我昨天遇到过的同一个人。
This is the same museum that you once visited.
注意：the same…that…结构中的that不能用as代替，因为the same…as…与the same…that…的含义不同。
（9）一个句子中有两个定语从句时，如第一个从句中已用who，第二个从句中宜用that。如：
The man who is at table is his brother that has been serving in
the army.
桌边的那个人是他曾当过兵的哥哥。
（10）当一个句子中含有两个定语从句时，如前一个已用关系代词which，后一个关系代词宜用that。如；
I’ll borrow a book which tells about the heroic deeds that the
PLA did in the battles against the invaders.
但两个定语从句的结构如果平行，一个定语从句用关系代词that，另一个定语从句也应重复that。如：
He told me to read a book that is very short, and that is very
interesting.
（1）who可以代表人、人格化了的动物、神话故事中的人物或有生命的事物。如：
The dog who is barking is our pet.
Those who want to go sign their names here.
（2）在从句中，作主语倾向于用who。如：
The girl who danced beautifully gave us a performance that day.
Do you know the comrade who spoke at the meeting just now?
（3）当先行词泛指代词he，they或指示代词those等时，常用who。如：
He who does no work gets no pay.
We are badly in need of those who can work in real earnest.
（4）先行词前出现不定冠词时，多用who。如：
I have found a man who can do this work.
He is a man who should learn from others.
（5）在there be句型中，既作主语又表示人时多用who，或省略。如：
There is someone outside the office who wants to see you.
There’s a man who lives in that village.
（6）不定代词one, ones, anyone等作先行词时，多用who。如：
One who works without complain is welcome here.
（7）定语从句和先行词分隔，用that可能引起歧义时，应用who。如：
I saw a man in the street who was surrounded by many people.
这个句子如用that，就可能被人认为是定语从句修饰street.
（8）在非限制性定语从句中，一般要用who，不用that。如：
The soldiers, who may have felt sorry for the boy, had him stand
with his back to his father.
（9）有两个定语从句出现在一个句子中，第一个关系代词用that的话，第二个就用who。如：
The man that I like is the one who is both competent and diligent.
但在平行结构中，应根据平行结构的原则重复同一个关系代词。如：
I met a Greek who traveled a lot in the world, but who knew very
little about his own country.
注意：关系代词前如有介词或在非限制性定语从句中作宾语，关系代词要用whom。如：
In the dark street, there wasn’t a single person to whom she could
turn for help.
The brave man, whom the tiger was shot by, is a good hunter.
（1）如果先行词是that，关系代词应用which。如：
What was that which you said about him?
你所说的关于他们的都是些什么呀？
（2）先行词和定语从句之间被其它较长的成分分隔，用which较好。如：
I have some interests outside my professional work which give
me great pleasure.
我有一些本职工作以外的兴趣，这给了我很大的乐趣。
（3）一个句子中如有两个定语从句，第一个定语从句用了关系代词that，第二个从句宜用which。如：
This is the book that you borrowed which you have lost.
这就是你借过又丢了的那本书。
但在平行结构中，应重复同一个关系代词。如：
I have a house which is located on the hillside, which faces the
south.
我有一座坐落在山边的面朝南的房子。
（4）非限制性定语从句中关系代词一般要用which。如；
I will buy a book, which tells about the use of English idioms.
我要买一本书，是讲英文习语用法的。
（5）相当于并列句和状语从句的定语从句，用which引导。如：
We went to the nearest port which we reached safely.
我们去了我们能安全到达的最近的港口。
He persisted in having a bicycle which he actually had no use
for.
他坚持要拥有一辆自行车，尽管他压根儿也不用。
（6）关系代词前如有介词，关系代词要用which。如：
They had a meeting at which he spoke on the current economic situation.
他们开了一个会，会上他谈到了当前的经济形势。
如把介词移至句末，可用that（或省略）。如：
This is the book (that) she was looking for.
这就是她在找的那本书。
关系代词as和which的选择是指它们引导非限制性定语从句的区别。尽管都可以用来引导非限制性定语从句，代表整个主句的意思，但两者的用法上是有区别的。as引导的非限制性定语从句常用来表示说话人关于某事的依据、态度、解释或评论等，含有“正如……”的意思。as从句放在主句的前面、中间或句末都可以。如：
She is working hard, as everyone can see. 或：
As everyone can see, she is working hard.
She married him, which was natural.但不能说：Which was natural,
she married him.
