it 在英语中的意思较多，用法较广，现总结如下。
(一)it作句子的真正主语
1.it 指前面已经提到过的人或事物，有时指心目中的或成为问题的人或事物，作真正主语。
例如：
What’s this? -It is a sheep? 这是什么??这是一只绵羊。
Who is it? -It’s me (I). 谁??是我。
It’s the wind shaking the window. 是风刮得窗户响。
2.it指时间、季节。一般用在无人称动词的主语。
例如：
What time is it? -It’s nine. 几点了??九点了。
It’s time for the meeting. Let’s go. 开会的时间到了，我们走吧。
What day is today? -It’s Saturday. 今天星期几??今天星期六。
What’s the date today? -It is October 1st.
今天是几号? ?今天是十月一日。
What season is it? -It is summer. 现在是什么季节??是夏季。
3.it 指气候。一般作无人称动词的主语。
例如：
Is it cold in this room? -No, it isn’t. 屋里冷吗??不冷。
What’s the weather like today? -It is fine.
今天天气怎么样??是晴天。
It often rains in summer and it often snows in winter in this city.
这个城市夏天经常下雨，冬天经常下雪。
4.it指距离、情况等。一般用作无人称动词的主语。
例如：
It is five kilometers from my home to the school.
从我家到学校有五公里。
It is very near from this factory to that one.
从这个工厂到那个工厂非常近。
It is a long way to the sea. 离海很远。
Is it well with you? 你身体好吗?
(二)it作形式主语
动词不定式、动名词短语起、从句在句子中起主语作用，而这一部分用词较多时，可以用it作为形式主语，放在句首代表其后所说的事实上的、真正的主语，而把真正的主语放在后面。
1.It +谓语+动词不定式。It 作形式主语，动词不定式作真正的主语。
例如：
It is difficult to climb a mountain. 爬山是很艰难的。
It’s a good habit to do morning exercises. 作早操是个好习惯。
It’s important to do proper memory work in the study of a foreign language. 在学习外语时适当地做一些有助于记忆的练习是很重要的。
It is right to do so. 这样做是对的。
2.It+谓语+动名词短语。It 作形式主语，动名词短语作真正的主语。
例如：
It is dangerous playing with fire. 玩火是危险的。
It is no use learning without thinking. 学而不思则罔。
It’s useless arguing with a silly boy. 和笨孩子争论是没有用的。
3.It +谓语+名词性从句。It 作形式主语，以that 引导的名词性从句是真正的主语。
例如：
It is a pity that you didn’t see such a good film.
你没看这么好的电影，真可惜。
It is certain that we shall succeed. 我们一定会成功。
It is strange that nobody knows where he lives.
真奇怪，谁也不知道他住在哪里。
(It is strange that…后面可用虚拟语气。
如：It is strange that he should have made such a mistake.
他居然犯了这样一个错误，真奇怪。
It is strange that nobody should know where he lives.居然没有人知道他住在哪里，真奇怪。)
It is said that the plane will take off at ten tomorrow morning.
据说飞机明天上午十点起飞。
(三)it作形式宾语
it 作形式宾语，代表其后所说的真正的宾语。真正的宾语是以that 引导的名词性从句或不定式短语。例如：
I consider it wrong that you Chinese students learn English without comparing it with your own language.
我认为你们中国学生学习英语，不和你们自己的语言比较是不对的。
I find it not so difficult to learn a foreign language.
我发现学习一门外语不那么困难。
I remember I made it clear to you that I was not coming.
我记得向你明确表示过我不来。
They want to make it clear to the public that they are doing an important and necessary job.
他们要向公众表明，他们在做一件重要而又必要的工作。
(四)it 用于强调结构
在表示强调的结构中，it 可用作先行代词，这种结构的句型如下：It +is/was +被强调的部分+that +其他部分。如果强调的部分是人，可用who, whom 代替that.
例如：Professor Wang teaches us English every Monday afternoon.王教授每星期一下午教我们英语。
强调主语：It is Professor Wang who teaches us English every Monday afternoon.
强调间接宾语：It is us whom Prof. Wang teaches English every Monday afternoon.
强调直接宾语：It is English that Prof. Wang teaches us every Monday afternoon.
强调状语：It is every Monday afternoon that Prof. Wang teaches us English.
It was here that I first met him. 这就是我初次与他见面的地方。
(强调状语)
It is the people who are realy powerful.
