It 在英语中使用频率也特别高，“it”是一个代词，可以用来代替某个事物、情况、概念等等。

it, one, ones

“it”通常是用于代替前面提到的特定事物或情况，而”one”通常是用于泛指某个事物或情况，“ones”是“one”的复数形式。

"It"是一个指代代词，用于代替前面提到过的事物或情况，或者作为形式主语或形式宾语。它通常不具有人称和性别的含义。
例如：
I bought a storybook yesterday. It’s very interesting.（这里的It代指前文提到的书）
"It is raining outside."（表示天气,“it”指代的就是“下雨”这个情况。） 
"I find it difficult to learn Chinese."（这里的ti代指学中文很难这件事情）

"One"是一个指示代词，通常用于代替前面提到的某个事物或情况，并且通常带有泛指的含义。它也可以用作形容词或代表数量的词。
例如：
"I like this book, but the other one is better."（这里的one表示另一本书） 
"One should always respect others."（这里的one表示一个人，泛指）
--- Who has a pen?--- I have one. You can use it.（这里的one表示“同类不同物”）
Your pens wrtie well, I want to buy ones.(这里的ones表示复数，相当于some)

It作形式主语

1) It is＋adj.+ for sb＋to do sth. 这里的形容词用来形容后面“to do”这件事情，此类形容词有：simple，easy, hard, difficult, possible, impossible, surprising, interesting, important, necessary,good, bad, exciting等。
如：It’s important to exercise regularly.
It was difficult for them to finish the task in such a short time.
It is necessary for us to keep learning in the modern world.
2) It is＋adj.＋of sb.＋to do sth. 该句型中，形容词只能用来描述某人的品德、特征，如：kind, rude, wise, foolish, silly, polite, impolite, friendly, clever等。
如：How rude it was of the boy to jump the queue!
It is polite of the boy to offer his seat to the old man.
3) It takes sb. … to do …. 意为“花费某人多长时间去做某事”
如：It takes much time and effort to finish the task.
4) It is no use/fun/good doing …. 意为“做某事是没有用/乐趣/好处的”
如：It is no use crying over spilt milk.
5) It is clear (true, possible, certain⋯) that ⋯ 常译为“（清楚，显然,，真的， 可能，肯定 ⋯⋯) ”
如：It is very clear that we can’t live with water so we must try our best to protect it.
6）It is said ( reported, believed, thought, learned ⋯) that ⋯ 常译为“据说( 据报道， 据悉 ⋯⋯) ”
如：It is reported that 10 people died in the accident.
7) It is a pity ( a shame ⋯) that ⋯ …… 真遗憾！……真可惜！
如：It is a pity that such a thing (should) happen in your class.
8) It is ( has been) ⋯ since ⋯ 自从……以来有多多长时间了
如：It is (has been) 5 years since he went abroad.
9) It happens ( seems, looks, appears ) that ⋯ 意为“碰巧(看来, 看起来）
如：It happened that he came across his teacher in the street.
It seems that he can’t finish the task on his own.

It 作形式宾语

基本结构为“动词＋it＋adj,＋不定式”，此类动词通常为think, find, feel, make, believe, consider等。
如：Planes make it easy to travel around the world.
I consider it necessary to do more reading.
I think it important to learn English by speaking.

I find it difficult to learn a new language.

It强调句：It is/was…that…

有时候我们会用强调句的结构来强调某个事物或情况，这时候“it”也会出现在句子中。

例如：

“It was Kate who broke the vase.”（是凯特打碎了花瓶。）
It was Mary who won the first prize in the competition.（是玛丽赢得了比赛的第一名。）

Mind map（It is adj. for/of sb to do）

例句

关于It的用法，其实也可以多读读，读得多了，自然也就有些语感了。

It's raining outside, so don't forget your umbrella.
I find it difficult to understand his accent.
It was a wonderful party, thanks for inviting me.
It is important to eat a healthy diet and get enough exercise.
It took me a long time to learn it, but now I can play the guitar.
I can't find my phone, have you seen it?
It's not my fault that we missed the bus, it was running early.
I think it's time we left the party and went home.
It's clear that you didn't study for the exam, your score was very low.
It seems like everyone is busy these days, it's hard to find time to get together.