主语是相对谓语而言的，主语相当于句子的主题，谓语说明主语的情况、提供新的信息。英语句子的语序一般是主语位于谓语之前，但实际应用时，主谓倒置的现象也是常见的。

主语通常是以名词词组或者相当于名词词组的结构表示的。

1. 名词作主语 [举例](1)The hunter killed the lion. 猎人杀死了狮子。

2. 代词作主语 [举例] I am hungry. 我很饿。

3. 数词作主语 [举例](1)Two-thirds of the students are boys in our school.

4. the+形容词/v-ed/v-ing结构作主语。
某些形容词与定冠词连用可以表示一类人或一抽象概念。
某些"v-ed分词或v-ing结构，其前用the也可以表示一类人或物. (常用复数)

[举例]

(1) The young should respect the old (the young，the old指一类人)
(2) Medical workers rescue the dying and heal the wounded. 救死扶伤。
(3) The wounded have been sent to hospital and the lost are being searched for. 伤者已被送到了医院，失踪者正在搜寻。

5. 不定式作主语

不定式结构直接置于句首的形式是最常见的，但使用时多数采用先行词it，而把不定式结构后置。

[举例]

(1) To remember to switch off the electricity is important.
(2) It is a pity not to go to the concert with you.
(3) It took US five hours to get here. 我们花了五个小时到这里。

6. v-ing结构作主语

v-ing结构直接置于主语的位置作句子的主语，也可用it作形式主语，而把v-ing结构置于句末。这种形式通常用于某些形容词或名词之后。

[举例]
(1) It is no good complaining.
抱怨是没有用的。
(2) It is no use arguing with him.
同他争辩没有用。

[注意]important，essential，possible，impossible，necessary等形容词不适于这种结构。

[举例]
(1) It is important learning foreign languages. (误)
(2) It is important to learn foreign languages. (正)
学习外语很重要。
[辨析]

作主语的v-ing结构与不定式结构在意义上是有区别的：
v-ing结构作主语通常表示抽象动作，而不定式结构作主语通常表示具体动作。

[举例]
(1) It's no use crying over spilt milk.
后悔没有用。(表示抽象动作)
(2) He realized that to go on like this was no use.
他认识到继续这样做没有用处。

7. 从句作主语

(1) that从句作主语(that从句作主语通常用先行词it作形式主语，而将that从句置于句末)，在这种结构中，从属连词that有时可省略。

[举例]
(1) It was obvious that the driver could not control his car.
(2) That he will succeed is certain.
他会成功是肯定的。

[注意]
(1) that从句放于句首时，that不能省略。
(2) wh-从句作主语

这种结构分两类：一类是由疑问代词或疑问副词引导，另一类由连词whether引导，前者是由特殊疑问句转化而来，后者由一般疑问句转化而来。

[举例]
(1) Who was responsible for the accident is not yet clear.
It is not clear who was responsible for the accident.
谁来负责这次事故不清楚。
(2) It is not known when he did it.
不知道他什么时候做的那件事。
(3) It is not important whether you are a male or a female in the job.
在这项工作中你是男性还是女性这不重要。