一、在英文中，名词或名词短语主要充当四种成分，主语、宾语、表语和同位语。如果我们把句子当作名词来使用，分别在另一个句子中作主语、宾语、表语和同位语(见下表第二列)，于是就构成了四种从句：主语从句、宾语从句、表语从句和同位语从句。因为以为这四种从句在本质上相当于名词的作用，所以我们把他们统称为名词从句。请看例句：
简单句 名词从句
“Jane Eyre” is great. What I am reading is great.
I believe his words. I believe what he said.
French is a useful tool. This BMW is what I want.
I like the car, Porsche. The mother must accept the fact that her baby is deaf.
二、三种句子来充当四种成分。(3，4)
上面说到名词从句就是用一个完整句子充当另一个句子的某种句子成分便构成了具体某一名词从句。
那么可以用什么样的句子来充当四种成分呢?答案是：
陈述句 Stuart is fat.
一般疑问句 Does he eat more everyday?
特殊疑问句 How much does he eat at home?
