一、只能用that作引导词的定语从句
1、先行词指物时,只能用that作引导词的定语从句：
1）先行词是指物的all,little,
few,much,any,anything,something,everything,
nothing,none,the one时.例如：
All that I can do has been done.我能做的都已经做了.
He won’t tell you anything that he hasheard about it.他不会告诉你他所听到的有关它的任何情况.
2）先行词前有all,little,few,much,any,every,no等修饰时.例如：
There is little work that is fit for you.没什么工作适合你做.
I have eaten all the food that is left.我把剩下的所有食物都吃了.
3）先行词时what或which时,为了避免重复,多用that引导.例如：
Which is the house that caught fireyesterday?昨天失火的是那座房子?
4）先行词是there be 句型中的主语,且指物时.例如：
There are some apples on the table thatare nice and sweet.桌子上有些非常甜的苹果.
5）先行词作主句的表语时.例如：
It’s a book that will help you a lot.这是一本对你有很大帮助的书.
It’s a sunny day that we are longing for.这是我们所渴望的晴天.
2、先行词指人时,只能用that作引导词的定语从句：
1）主句是以who开头的特殊问句时.例如：
Who’s the man that the teacher is talkingto?和老师谈话的这个人是谁?
3、不管先行词指人还是指物,都只能用引导词that的定语从句：
1）先行词是数词时.例如：
---I have two smart birds.我养了两只可爱的鸟.
---Are they the two that you bought lastweek?是你上星期买的这两只吗?
2）先行词被序数词以及last,next等修饰时.例如：
It’s the last bus that you can catch.这是你可以赶上的末班车.
He stopped the fourth thief who wasrunning away.他把准备逃跑的第四个小偷拦截下来了.
3）先行词是名词化的最高级形容词,或者先行词被最高级形容词修饰时.例如：
You can choose the best that you think.你可以挑选你认为是最好的.
This is the best film that I have seen.这是我看过的最好的影片.
He’s the finest men that I have everworked with.他是我共事过的最好的人.
4）先行词被the only,the
very,the last,the
just,the same 等修饰时.例如：
You’ve the only person that I met here.你是我在这儿遇见的唯一的一个人.
This is the very shirt that I am lookingfor.这正是我一直在寻找的衬衫.
5）引导词为定语从句的表语时.例如：
She’s not the girl that she used to be.她不再是过去的她了.
6）先行词同时包括人和物时.例如：
We often talk about the people and thethings that we are interested in.我们常常谈论我们感兴趣的人和事.
二、先行词指物时,只能用which作引导词的定语从句
1、引导先行词为事物的非限制性定语从句时.例如：
My dog ,which is now very old,became ill yesterday.我家的狗昨天病了,它已经非常老了.
The pen,which writes verywell,cost me RMB 20.这支笔花了我20元,它非常好写.
2、关系代词前有介词时.例如：
This is the classroom in which westudied last year.这是我们去年在里面学习的教室.
Have you ever been to a farm onwhich there is a small zoo?你曾经到过有个小动物园的农场吗?
三、先行词指人时,只能用who作引导词的定语从句
1、关系代词在从句中作主语时,通常用who.例如：
The boy who helped you is myyounger brother.帮你忙的那个男孩是我弟弟.
Is the woman who rides a red bikeevery day your aunt?每天都骑着一辆红色自行车那个女人是你姑姑吗?
2、先行词为指人的all,one,ones,someone,anyone,everyone,no one等代词时.例如：
One who doesn’t work hard willnever get happiness.不努力工作的人,永远得不到幸福.
Do you know anyone who can mendlocks?你认识会修锁的人吗?
3、先行词是there be句型中的主语,且指人时.例如：
There is a stranger who wants tosee you.有个陌生人想要见你.
Once there was king who was verysilly.从前,有个非常愚蠢的国王.
4、在非限制性定语从句中.例如：
His father,who was ill inhospital for a year,died yesterday.他父亲住了一年的院,昨天过世了.
I know the woman very well,who often wears a redT-shirt.我非常了解那个妇女,她经常穿着一件红体恤衫.
5、在被分隔的定语从句中.例如：
I know the woman very well whooften wears a red T-shirt.我对经常穿着一件红体恤衫那个妇女非常了解.
A new teacher will come tomorrowwho will teach you German.教你们德语的新老师明天就会来.