1.who / that 在定语从句中做主语时,
谓语动词的单复数应与先行词保持一致
I prefer shoes that are cool.
I like a pizza that is really delicious.
I love singers who are beautiful.
I have a friend who plays sports.
2.( who 和that 引导定语从句作主语 不可省略）
The old man is a professor.He teaches chemistry in a college.
The old man is a professor who teaches chemistry in a college.
The old man is a professor that teaches chemistry In a college.
3.(which 和that 引导定语从句作宾语 可省略）
I enjoyed reading the book.You gave me the book.
I enjoyed reading the book you gave me .
4.(whose 在定语从句中作定语用）
What’s the name of the man?His sister is a doctor.
What’s the name of the man whose sister is a doctor?
Have you seen the cup?Its color is red.
Have you seen the cup whose color is red?
5.when 在定语从句中作时间状语
I‘ll never forget the day .I first came to Beijing.
I‘ll never forget the day when I first came to Beijing
6.where 在定语从句中作地点状语
This is the cinema.I watched Harry Potter last month in it.
This is the cinema where I watched Harry Potter last month.
注意：（1）当定语从句中含有介词,介词放在句末时,who,that,which可省略,但介词在关系代词前时,只能用“介词+which/whom”结构.例如：
This is the house in which we lived last year.
这是我们去年居住的房子.
Please tell me from whom you borrowed the English novel.
请告诉我你从谁那借的这本英文小说.
（2）含有介词的固定动词词组中,介词不可前置,只能放在原来的位置上.
例如：This is the person whom you are looking for.
这就是你要找的那个人.
先行词被序数词或形容词最高级所修饰,或本身序数词、基数词、形容词最高级时,只能用that,而不用which.例如：
He was the first person that passed the exam.
他是第一个通过考试的人.
b.被修饰的先行词为all,any,much,many,everything,anything,none,the one等不定代词时,只能用that,而不用which.例如：
Is there anything that you want to buy in the shop?
你在商店里有什么东西要买吗
c.先行词被the only,the very,the same,the last,little,few 等词修饰时,只能用that,而不用which.
例如：This is the same bike that I lost.
这就是我丢的那辆自行车.
d.先行词里同时含有人或物时,只能用that,而不用which.例如：
I can remember well the persons and some pictures that I saw in the room.
我能清楚记得我在那个房间所见到的人和一些照片.
e.以who或which引导的特殊疑问句,为避免重复,只能用that.例如：
Who is the girl that is crying?
正在哭泣的那个女孩是谁?
f.主句是there be 结构,修饰主语的定语从句用that,而不用which.例如：
There is a book on the desk that belongs to Tom.桌子上那本书是汤姆
（5）关系词只能用which,而不用that 的情况：
a.先行词为that,those时,
What’s that which is under the desk?
b.关系代词前有介词时,例如：
This is the room in which he lives.
c.引导非限制性定语从句,例如：
Tom came back,which made us happy.汤姆回来了,这使我们很高兴.
限制性定语从句紧跟先行词,主句与从句不用逗号分开,从句不可省去,非限制性定语从句主句与从句之间有逗号分开,起补充说明作用,如省去,意思仍完整.