如何判断英语句子中的语法成分之主语
主语是英语句子中一个主要语法成分,常出现在句首（但出现在句首的并非总是主语）.这一讲我们将学习英语句子主语的形式选择、意义选择和主谓一致三个方面.
一、主语的形式选择
主语表示句子描述的是“谁”或“什么”,是谓语的陈述对象,由名词性的成分来充当,说明人物、地方、行为、事件、事实等.英语中除了名词和代词（47页例1-3）之外,名词性的成分还包括：
（一）“定冠词the+形容词”,可起名词作用,表示某一类的人或事物.
The blind often have sharp hearing.
The wise are those who know themselves.
The Chinese love peace.
The good in him outweighs the bad.
（二）动词的非谓语形式（不定式或动名词）可起名词作用.
在英语中,当动词不是用作句子谓语,而是用于充当其他语法功能的时候,动词就会以非谓语形式出现,主要有三种：不定式（Infinitive）、分词（Participles）和动名词（Gerund）.其中,可以充当主语的非谓语形式是不定式和动名词.
1、不定式作主语
（1）当不定式作主语时,如果较短,可直接出现在常规的主语位置,如：
To leave or stay is a question hard to answer. To say something is one thing, to do it is another.
（2）但是如果不定式充当的主语较长而谓语较短时,常用“it”作形式主语,而把真正的不定式主语放在谓语的后面.
It’ll be a crime to stay anywhere else.
It’s always a good ides to get a second opinion.
It would be wrong not to take this problem into consideration.
（3）有时,不定式的前面可以有一个连接词、连接代词或连接副词（包括whether, what, which, whom, where, when, how）.不定式的这种结构在句中主要起宾语作用,如：
He will tell us how to use the library. 他将告诉我们如何使用图书馆. 但也可以作主语,如 What to buy for dinner is not decided yet.
Where/Where/How/Who to prepare the dinner is not decided yet.
2、动名词作主语
（1）动名词与现在分词同形,但主要起名词作用,在句中可充当主语,如课本47页例5：
Learning English grammar is by no means easy. Seeing is believing.
Reading French is easier than speaking it.
（2）在一些习惯表达中,常用形式主语“it”,而将作主语的动名词短语放在后面,多见于某些形容词或名词之后,其他情况一般不用it作形式主语代替作主语的动名词.如：
It is no use telling him not to worry.
It is no good talking about that.
It is worthwhile trying again.
It is a water of time trying again.
（3）当动名词有自己的逻辑主语时,常可以在前面加上一个名词或代词的所有格,构成动名词的复合结构（这时,名词或代词的所有格做动名词的逻辑主语）.动名词的复合结构也可以在句中作主语.例如：
Their coming to help was a great encouragement to us.
*动词不定式和动名词都可以用作主语.在意义上相近.但动名词多用来表示泛指或抽象动作,不定式多用来表示特指或具体动作.比较：
Smoking is not good for health. It is not good for you to smoke so much
（三）主语从句
1、句子也可以起名词作用,在句子中充当主语,形成主语从句.判断主语从句时,可先找到句子的谓语,谓语前面的从句就叫做主语从句.如
Who murdered Joan/Why Jack murdered Joan/When Jack murdered Joan remains unknown.
Why he left wasn't important.
That she became an artist may have been due to her father's influence.
2、借助形式主语it,主语从句放在后面.
（1）由that引导的主语从句可以放在主语的位置,这时that不可以省略.例如：That prices will go up is certain.
that主语从句放在句首的情况不是很常见,绝大部分主语从句都借助形式主语it,主语从句放在后面.如：
It is strange that he knows nothing about it. It is a pity that he can't swim.
It happened that he wasn't in that day.
It is said that there has been an earthquake in India.
在口语中,用it做形式主语时,主语从句的that可以省略.
（2）wh-疑问词引导的主语从句,可以放在句首（第一组）,也可以借助形式主语it（第二组）.
第一组：When he'll be back depends much on the weather.
How it was done was a mystery.
Whether we'll succeed remains to be seen.
第二组：It is uncertain whether the game will be held.
It's a puzzle how life began.
It doesn't matter much where we live. Is it known where he went?
注意：疑问词后面的语序应为正常的陈述语序.
二、主语的意义选择
除了形式上的多样性之外,英语句子的语义类型也是多种多样的,包括动作的执行者、动作的承受者、动作得以完成的工具、事件的原因、地点、谈论的话题等等.
Jack murdered Joan with a gun.
Joan was murdered by Jack with a gun. A gun killed Joan.
Cancer deprives many people of their lives.
The hall can seat 50 people for a conference.
Learning Chinese Kongfu is by no means easy.
在实际的语言使用中,我们可以根据自己的意图来选择主语的意义类型.一般来说,主语通常总是某种已知给定的信息,是后面谓语部分的评述对象.
The bees swarmed in the garden. The garden swarmed with bees.