他们词性不一样，自然就有很多区别
首先他们都是将句子用作词使用，即词化
句子的词化
句子是一个相对完整独立，表达一层含义的语言片断，句尾有标点符号。但有时根据需要，要求一个句子用作名词/形容词/副词使用。那么这个用作名词/形容词/副词使用的句子就必须要名词化/形容词化/副词化。将句子名词化/形容词化/副词化要在该句句首加上一个相应的词化标志，加上相应的词化标志之后该句就可用作名词/形容词/副词使用了。名词化/形容词化/副词化之后的句子与名词/形容词/副词在句中的位置基本一致。
1．将句子名词化（主语从句/宾语从句/表语从句/同位语从句）
⑴将句子名词化首先要看句中是否有what /which /where /when /whose/who /whom /why /how九种疑问意的词,若句中有这九种疑问意的词则将句子名词化就是将这九种疑问意的词作为词化标志提至句首。
例;They are going to where名词化后为where They are going
⑵若句中有是否意的词whether/if(if慎用)则将是否意的词whether/if放在句首，作为词化标志。
例：I don’t know whether he whether will come.
注：if 引出的名词性从句只能做及物动词和形容词sure/ certain等的宾语。
⑶若句中即没有疑问意的词也没有是否意的词则在该句句首加that作为词化标志。
例：I don’t know that he will come tomorrow.
The problem is+表语从句 1.What he want +is a problem.主语从句
2.which book he wants
3.where he will go
4.when he will go
5.who will go there
6.whose father will go there
I know+宾语从句 7.whom he will go with
8.why he will go there
9.how he will go there
10.whether he will go there
11.if(慎用) he will go there
12.That the earth is becoming polluted
易错点 ①你认为你在哪？
Do you think where you are？ ×
Where do you think you are?※√
② 1.(我)恐怕它不能来=2. (我)它恐怕不能来=3.(我)我恐怕它不能来
I am afraid that he can’t come.
③同位语从句署名词性从句，他用来揭示.阐明一个名词的内容。
经常跟有同位语从句的名词有：idea想法/news消息/fact事实/report报导/suggestion建议，等。
④What……从句：在what引出的从句中，what除了当“什么”讲之外还可以表达“所……的”
What the people need is what we need.人民所需要的就是我们所需要的。
⑤表示疑问意的词后面加ever也可以构成词化标志如：
whatever无论什么/ wherever无论何处，等。
2.将句子形容词化（定语从句）
用一个句子作定语来说明一个名词，这个被说明的名词叫“中心词”。将句子形容词化首先要找出该句中与中心词相同的词（我们称这个词为替代中心词）,然后将这个替代中心词改成相应的代词提至句首，作为词化标志，使这个句子形容词化，作定语使用。
将句子形容词化具体方法如下：
① 若代替中心词指人，应将代替中心词改成who/that/[whom(宾语)]
② 若代替中心词指物，应将代替中心词改成that/which
③ 若代替中心词指所属关系，应将代替中心词改成whose/…of which
例：⑴I saw the man this morning. You met the man in the street yesterday.
I saw the man who/that/whom you met in the street yesterday this morning.
⑵This is the room. Luxun lived in the room.
This is the room which Luxun lived in. (不可省略)
注：从句还可以出现下列形式即介词+词化标志，当该结构在从句中作状语时，可以使用where/when/why来代替该结构表示地点/时间/原因
例：This is the room in which Luxun lived.
=This is the room where Luxun lived.
有关定语从句的几点说明:
1. 限制性和非限制性定语从句
① 限制性定语从句：以前我们所学过的定语从句都是用来说明一个名词的情况，他与中心词关系紧密总是位于中心词之后。
例：His son who works in Shanghai will come back. (他的在上海工作的那个儿子回来了。)
② 非限制性定语从句：非限制性定语从句与中心词关系不紧密，只是一种附加说明，如果去掉也不影响句子的含义。非限制性定语从句有时于中心词用“，”隔开或放在句尾。
例： His son, who works in Shanghai, will come back.
2. 在下列情况下中心词指物时，替代中心词只能用which不能用that:
① 介词加词化标志指物时，不能用that 只能用which
例：This is the factory in which I worked for three years.
② 在非限制性定语从句中不能用that只能用which
例：I said nothing, which made him still angry.
③ 当中心词本身就是that时，为了避免重复，词化标志不用that只用which
例：He had found that which he was looking for.
3. 在下列情况下词化标志只用that不用which或who
① 当中心词是人加事物时
例： They talked about the teachers and the schools that they had visited.
② 当中心词被最高级/last/next/only/very修饰时，词化标志不用which和who而用that
例：This is the best film I have ever seen.
③ 当中心词被序数词修饰时，词化标志不用which和who而用that
例：This is the first letter that I received from her since her left.
④ 当词化标志在从句中作表语时，词化标志不用which和who而用that
例：China is not the country that it used to be.
⑤ 当中心词是anything/everything/nothing或all（指事物不指人）{注：没有something}时，词化标志不用which和who而用that
例： I will tell you everything that I know about it.
⑥ 为了避免重复时，词化标志不用which和who而用that
例：Which is the car that hit the boy?
Who that has such a horse doesn’t love it?
4. as也可以引出定语从句代替which代替一句话或一个名词。As引出的定语从句一般是非限制性定语从句，也可以放在句首，但which引出的非限制性定语从句不能放在句首。
例：She is very careful as/which her work shows.
=As her work shows, she is very careful.
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