急！关于定语从句的几个问题

1.what 引导名词性从句，如主语从句，宾语从句，表语从句
What he wants to tell us is not clear. 他要跟我们说什么，还不清楚。（主语从句）
I don’t know what he wants.我不知道他要什么。（宾语从句）
That’s just what I want. 这正是我想要的。（表语从句）

2.that既可代表事物也可代表人，which代表事物。which, that它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，例如：
(1) A prosperity which / that had never been seen before appears in the countryside. 农村出现了前所未有的繁荣。（which / that在句中作主语）
(2) The package (which / that) you are carrying is about to come unwrapped. 你拿的包快散了。(which / that在句中作宾语）

代表物时多用which，但在下列情况中用that而不用which：
a)先行词是anything, everything, nothing , none等不定代词时；
b)先行词由every, any, all, some, no, little, few, much等修饰时，这时的that常被省略；
c)先行词前有序数词或形容词最高级修饰时； d)先行词就是序数词或最高级时；
e)先行词中既有人又有物时；
f)整个句中前面已有which时。

which指物
在定语从句中做主语或者宾语，做宾语时可省略。
(1) Football is a game which is liked by most boys. 足球是大多数男孩都喜欢的游戏. (2) This is the pen (which) he bought yesterday. 这是他昨天刚买的钢笔.

that指人时相当于who或者whom；
指物时，相当于which。
在定语从句中做主语或者宾语，做宾语时可省略。
(3) The number of the people that/who come to visit the city each year rises one million.
(4) Where is the man that/whom I saw this morning?

3.the one 是特指，必须代替的是前面的名词。
the one和that在词性上都是代词。但是，the one在定语从句中是代替人或物的（He is the one who always laugh at me.)，而that则是可以在人或物做主语时做关系代词，（He likes the apple that is red.)

当两个词在一起时，可以这样用
I am the one that likes to sing.

the one 用来代替前面特指的单数名词,有时可用that代替(尤其在有后置定语的情况下.
that=the+单数可数名词/不可数名词 代替前面出现的单数可数名词/不可数名词
the book on the desk is better than /the one under the desk
the populatin of china is larger than that of america

但在下列情况中，只能用 that，不用 which：
①当先行词是 all, a lot, (a) little, few, much, none, anything, something, everything, nothing 等词时。如：
All that can be done has been done.
In this factory I saw little / much that was different from ours.
②当先行词被 all, any, no, much, little, few, every 等限定词所修饰时。如：
We heard clearly every word that he said.
③当先行词是序数词或被序数词所修饰时。如：
The first thing that should be done is to get the tickets.
When people talk about Hangzhou, the first that comes to mind is the West Lake.
④当先行词是形容词最高级或被形容词最高级所修饰，以及先行词被序数词和形容词最高级同时修饰时，如：
Is that the best that you can do?
That’s the most expensive hotel that we’ve ever stayed in.
This novel is the second best one that I have ever read.
⑤当先行词被 the very, the only, the next, the last等所修饰时。如：
This is the very book that I want to find.
⑥当先行词为指人和指物的两个并列名词词组时。如：
The guests spoke highly of the children and their performances that they saw at the Children’s Palace.
She described in her compositions the people and places that impressed her most.
⑦当主句是以 which 开头的特殊疑问句时。如：Which is the car that killed the boy?
介词前置时which，非限定性定语从句用which
the one 后面通常用that