定语从句和名词性从句如何区分？

最关键的一点：名词性从句包括主语从句、宾语从句、表语从句和同位语从句，这些从句分别在整个句子中作主语、宾语、表语和同位语。而定语从句在整个句子中作定语，起修饰作用。

具体用法如下：

名词从句
名词从句有四类：宾语从句、主语从句、表语从句、同位语从句。例如：
宾语从句：I don’t know where he will go.
主语从句：Where he will go is unknown.
表语从句：The problem is how we can get so much money.
同位语从句：We all feel sorry about the news that he failed in the game.
下面几点是学习名词从句要加以注意的：
1． whether和if都可以引导宾语从句表达“是否”意，（即引导宾语从句时if和whether
可以互换）但以下几种名词从句要用whether（不用if）引导：
（1） 引导主语从句，且在句首时。例：Whether he has won the tennis is not known.(但 It is not known whether/if he has won the tennis).
（2） 引导表语从句时。例The problem is whether he can get a job.
（3） 引导同位语从句时。例：I have no idea whether he will come.
（4） 介词后的whether从句。例：I worry about whether I hurt her feelings.
2． 位于句首引导主语从句的that不可忘。
例如：That he got the first place in the competition surprised all of us.
3． 在“It is suggested/proposed/ordered/a pity/no wonder/necessary/strange/…＋
that从句”结构中，that从句谓语有时用（should）do这样的虚拟语气形式。
例如：It is suggested that we(should) improve the relations between us.
It is a pity that one(should) stay in one place all one′s life.
It is strange that she should have failed to see her own shortcomings（缺点）.
4． 在“The reason why…is that…”句式中that不要误为because.
例如：The reason why he didn′t go to school yesterday is that he was ill.
5． 名词从句中that,what用法比较：
引导名词从句的that是连词，在句中无成份，无意义，而what是连接代词，what＝the thing(s) that.
例如：It was told in yesterday′s newspaper that what the students had done was praised by the factory.
I know that he will study.
I know what he will study.
That he works hard at his lessons is known to us all.
What he works hard at is known to us all.
All I ask is that you should tell the truth./He is not what(＝the person that)he
used to be.他和以前大不相同了。
6． where在名词从句中的使用特点：
where在名词从句中有时可以变为“the place where”,有时可以变为“介词＋the place where” 形式。例如：
主从：Where(＝The place where)she has gone is still unknown.
宾从：Would you please tell me where(＝the place where) Mr Smith lives?
表从：Your dictionary remains where(＝in/at the place where)you put.
同位从：Have you any idea where(＝of the place where) she is spending her holidays?
7． 无论是哪种名词从句都是陈述语序。例如：
I know where he lives./Please tell me what her name is.
当一个特殊疑问句本身就是陈述语序（即“主＋谓＋（宾）”或“主＋系＋表”结构），将其变为间接引语（即宾语从句）时，无需改变语序。例如：
What′s the matter with him? She asked me what was the matter with him.
Who looks after your grandfather? He asked me who looked after my grandfather. ===========================================================================
定语从句

一、 定语从句与引导词
定语从句是在复合句中修饰某一名词或代词的从句。被修饰的名词或代词叫做选行词，
而引导定语从句的词叫引导词，有关系代词和关系副词两类。
懂得什么样的先行词后用什么样的引导词是学好定语从句的关键。
1． 关系代词有：that,which,who,whom,whose,在定语从句中作主语、宾语中表语，whose 作定语。代人的有：who,whom,whose,that；代物的有：that,which,whose.例如：
The man who helped you is Mr White.(who代人，在定语从句中作主语)
That is the person(whom/who/that) you want to see.(whom,who,that代人，在定语从句中作宾语，可省略)
I′m not the fool(that) you thought me to be.(that代人，在定从中作表语)
A dictionary is a useful book which(＝that) tells us the meaning of words(which或that代物，在定从中作主语)
2． 关系副词有：when(指时间)，where(指地点),why(指原因)，在定语从句中作状语。
例如：Would you suggest a time(when) we can have a talk?(when可省略)
The house where they live is not very large./This is the reason why he did not came to the meeting.
注意：不是表时间的先行词都用when引导定从，不是表地点的先行词都用where引导定从。例如：We′ll visit the factory which(＝that) makes radios.
(which或that在定从中作主语，where不可用主语，故不可用)
They still remember the happy days(which/that) they spent in Beidaihe.
(which或that在定从中作spent的宾语，when不可作宾语，故不可用)

二、限制性定语从句与非限制性定语从句
限制性定语从句和主句之间不用逗号分开，修饰主句中的某一个名词或名词词组或代
词；非限制性定语从句常用逗号和主句隔开，可以修饰主句中的某一个词，也可修饰整个句子。非限制性定语从句不能用that引导，引导非限制性定语从句的关系词不能省略。非限制性定语从句例：Li Ping′s father, who works in a factory, is an engineer./He tore up my photo, which made me very angry.(which指代主句内容，因前后两句是因果关系，此时which不可换为as。)
He is good at physics, as is known to us all.＝As is known to us all, he is good at physics.(as指代主句内容，在as is known/believed, as we all know/believe这类结构中as不可换为which)

三、 只用that引导和不用that引导的场合
1． 只用that引导的场合
1） 当先行词既包括人又包括物时：He talked about the men and the books that attracted him.
2） 指物的先行词被any, every, only, very, all, no等修饰时：These are the very points that interest me./That′s the only watch that I like most.
3） 指物的先行词被序数词或形容词最高级修饰时：The first step that we are to take is very difficult./This is the second card that he gave me.
4） 先行词是指物的不定代词all,little,few,much,及everything,anything,nothing等时：There is still much that can be done about it./Have you got everything that you need?
5） 先行词是who时：Who that have seen him does not like him?
2． 不用that的场合如下：
1）非限制性定语从句中 Last night ,I saw a very good film,which was about the Long
March .
2）介词放在关系代词之前时 This is the man from whom I learnt the news
3）句中出现了that，或先行词是that时 I have found that which I was looking for.

四、“介词＋关系代词”用法
1． 介词的确定应依据定语从句中短语的习惯性搭配，例如：Who is the girl with whom you just shook hands?(shake hands with…是习惯性搭配)
2． 介词常受先行词的制约（即介词和先行词的搭配），例如：He built a telescope through which he could study the skies.(through which 即through the telescope)
3． 当关系代词作“动词＋介词”固定短语的宾语时，此时因定短语中的介词不能拆开移到关系代词前。例如：This is the watch which you′re looking for./He is a kind of man whom you can safely depend on.

五、几个名词后的引导词
1. situation后常用where,in which引导定语从句：Can you imagine a situation where/in
which you can use the word?
2. way(方式，方法)后常用in which或that或how引导定语从句，也可省略：Do you know the way(in which/that/how) he worked out the problem?
3. “the same＋名词”，“such＋名词”，“as＋名词”后通常用as引导定语从句。
例如：I have the same dictionary as you(have)./Such a book as you bought is not worth buying.