英语中可以充当主语的成分有：名词，代词，名词化了的动词，形容词，分词，副词或数词等，动词不定式或不定式短语， 从句，以及某些固定词组。

作主语的词：名词，代词，名词化了的动词，形容词，分词，副词或数词等，动词不定式或不定式短语， 从句，某些固定词组。
名词作主语
例：The villagers didn't realize how serious the pollution was until the fish in the river died.
　　直到河里的鱼死了，村民们才意识到污染的严重性
S（subject主语）+ V（verb谓语动词） +O（object宾语）. 主系表结构 :S（subject主语）+V（verb谓语动词，也称为系动词，就是所说的“系”）
倒装：Not until the fish in the river died did the villagers realize how serious the pollution was.
注意：倒装在主句，从句不倒装 /did 和not 提前，只剩动词原形
强调：It was not until the fish died in the river that the villagers realized how serious the pollution was.
如果强调的是主语或宾语且指人用who，其他一律用that。是that，而非其他/主句的顺序（倒装不强调，强调就不倒装）
人称代词（主格），名词性代词，指示代词，不定代词，such（----样的人、物），it（时间，天气，距离等）
at midnight /in the(泛指）middle of the night
例：It is 9 o'clock when I went home.
我回家的时候已经九点了
一般句：It was nine when I got home.
强调句：It was at nine that I got home.
原形：I got home at nine.
注意：作主语在句首/it作主语构成（常用 v-ing，不常用只有 use，good）
　　（1）It is +adj. +of sb.(人的属性）/for sb.（物的属性）to do
　　例：It is necessary to finish the design before National day. Tomorrow is National day.
　　有必要在国庆节之前完成设计。明天是国庆日
　　（2）it is +n.+for/of sb. to do
　　例：It is not good manners for Arabs to stand close talking to their friends.
　　对阿拉伯人来说，和他们的朋友说话离得近不是一个好习惯
　　It is a great honor for us to be invited to the party.
　　被邀请参加聚会是我们极大的荣幸
在感叹句中必须用it作形式主语：What a joy/ pleasure (it is) to read the book.
在问句中必须用形式主语：Is it possible to go to the moon by spaceship?：可以乘宇宙飞船去月球吗？
数词作主语
Ten out of 100 Americans are over /above /more than 65.
动名词作主语
一般 / 完成时
v-ing: 主动/doing/having done
not doing:被动/being done/having been done
注意：doing 无时间限定/与谓语同时出现/在谓语动词后发生having done发生在谓语之前
He still remembers being prized.
当逻辑主语为无生命的东西时
1.作主语时，通常表示一般性的无时间性的东西。而不定式表示某一种具体的动作。
Smoking is habit difficult to break：抽烟的习惯很难打破的
2.主语和表语一致
Seeing is believing：眼见为实
3.逻辑主语必须是所有格形式
The old man's taking pity on the snake leads to his death.那个老人怜悯那条蛇，导致他的死亡。
4.“it” 作形式主语：当表语为 no use，no good，no harm，any good，fun等名词或useless，nice，good，interesting，worthwhile等形容词时，句首常用it作形式主语，动名词后置。句型常用于说明“某种活动或某件事情如何”
不定式作主语
To +动词原形
Not to do 解释：具有动词的性质，作宾语，状语/具有名词的性质，作主语/具有形容词性质，作定语/具有副词的性质，作状语。除谓语不可作之外，其他都可。
一般式：to do
进行时：to be doing
完成时：to have done
完成进行时：to have been doing
to do:表示动作发生在谓语之后，I want to go home.与谓语动词几乎同时发生
to be doing :表示与谓语动词同时发生
to have done:发生在谓语动词之前 Someone was considered to have done something.
to have been doing :动作从过去开始一直持续到谓语动词这个动作的发生
不定式表示一种确定的动作
To do morning exercises this morning is not good for your health：今天早上去做早操对你的健康不好
名词化的形容词：the poor /the sick
名词化的过去分词：the disabled
名词化的介词短语：From my home to school is three kilometers：从我家到学校有三公里
副词----不定式或动名词
What an excitement to find a shark when you’re swimming in the sea.
当你在大海中游泳时，发现鲨鱼是多么兴奋的事
句子作主语
〔1）that引导的从句，无意义，不担当句子成分，不省略
　　a. It +v+ that从句
　　b. It +系动词+表语+that 从句
　　c. It is strange that he didn't come here yesterday：奇怪的是，昨天他没来这里
　　当形容词为important necessary strange /that should +动词原形
　　It is strange that he should （表示吃惊）pass the exam：奇怪的是，他可以通过考试
〔2）It is +n +that 从句
　　What a pity it is that you didn't come to this party：真遗憾你没来这个聚会
　　当名词为a pity/a shame /a surprise +should +v原形（表示竟然）
　　What a pity that she should be fail in exam：真遗憾，她竟然考试不及格
〔3） It is +过去分词+that 从句
　　It is reported that a car accident happened there：据报道，发生一场车祸
　　从句s +be v-ed + inf.
　　He is said to be studying in the USA：据说他在美国学习
　　有人建议suggest/order/request + 从句 should +动词原形
〔4） It +动词原形seem/appear /turn out/+that
　　It seems that it will rain：看来要下雨
　　〔1 〕It strikes sb. that 从句 ：表示“某人突然想…”
　　It occurs to sb. that从句：表示“某人发生了…”
　　〔2〕 为强调从句意义，that从句常放在句首
　　例：That he will come here is of great help：他来这儿是很有帮助的
　　That you failed the exam will…：你考试失败了会…
　　--What made his father so angry? ：什么使他父亲很生气？
　　--That he failed the exam. 或He fails the exam：他考试不及格
c.在问句中必须有形式主语
It is true that Bush will visit China again?
Whether /if是否不担当句子成分，也不省略
A.句首只用whether
Whether he will come to the meeting is unknown yet.
B.句末形式主语都可用 It ----whether/if
It is of no/little important whether/if it will rain today.
c.在有些句型中，从句不在前 It doesn't matter whether ----
It doesn't matter whether it is new/it will rain tomorrow.
〔3) 连接代词 who /whom/whose/what/which/担当从句成分，有意义，不省略
A. what +sb. + clause
What we will do next is not decided.
B. what+修饰词 意为什么样的……
What color you like is none of my business.
C. what ……人，……事，……物
What he said at the meeting surprised everyone present.
注意：what前是否有先行词/与that 之间的区别
That he said it at the meeting surprised everyone present.
What 与which 的区别
Which/what you want---若给定范围用which，没有用what。
What ---be that---强调句的一种（强调后者）：
What I need most is that someone helps me.
另外，what引导的从句作主语时，大多数情况下视为第三人称单数：
What we need is time.
What you said yesterday is right.
但也有例外，如What we need are good doctors.
注意：当what引导的名词性从句作主语时，主谓一致问题极为复杂。著名学者周海中教授在论文《关于what-分句作主语的主谓一致问题》中就这一问题做了深入研究，并给出了10种的主谓一致关系。这些一致关系值得英语学习者和使用者特别注意。
There be +what clause
There was we call People's Republic of China in Chinese history.
What +比较级what more /what worse
what引导从句作状语修饰全句的内容。在句首或句中，很少在句尾
Which +n (single)
Whose +n(只用此）
Whose father is a doctor is no sense.
It从句作形式主语（what……人或物时，不用it）
〔4）连接副词when/whose/how/why担当句子成分，有意义，不省略
How the accident happened puzzled everyone.句尾用形式主语it
〔5) 引导主语从句
whoever/whosever/whatever/whichever/Anywhere/anything that /everyone that/whoever=anyone whom/anything that (泛指)/anything what (特指)
Whoever 指人从意义判断
Who 引导指事
Since the introduction of reform, China has made earth shaking changes.自从改革以来，中国发生了翻天覆地的变化。