it是人稱代名詞，在句中用作簡單的指代、非人稱的指代、形式上的主語或賓語等。

用作簡單代名詞的it?當句子中的有些成分重復出現時，可以用IT代替前面出現過的名詞或其同等語、其他片語、子句或句子。

1. it代替前面出現過的名詞或其同等語。

I bought a new car last week, but it was stolen yesterday.

我上周買了一輛新車，但它昨天就被盜了。

it代替名詞片語a new car。

2. it代替前面出現過的其他片語。

I tried to calm him, but it was impossible.

我試圖使他平靜下來，但這不可能。

it代替不定詞片語to calm him。

3. it代替前面出現過的子句或句子，但用在動詞forget, know, remember, try等的后面時通常省略。

Your lottery wins the first prize. It will bring you a lot of money.

你的彩票中了頭獎。這會給你帶來很多錢。

It代替句子Your lottery wins the first prize。

用作先行代名詞的it

用作先行代名詞的it主要充當形式上的主語或賓語。

1. it作形式上的主語或賓語時，后面真正的主語或賓語通常是不定詞片語、動名詞片語或名詞子句。

It is true that he won the top prize.

他得了大獎是真的。

名詞子句that he won the top prize是真正主語。

2. it在"It seems (appears, looks, chances, happens) + that (as if)..."的結構中作主語時，沒有具體意義。

It seems that we should ask him to give us a treat.

似乎應該讓他請客。

3. it作形式主語時后面可以接is+過去分詞+that引導的名詞子句。如：

It is believed that(據信)

It is reported that(據報導)

It is well known that(眾所周知)

It is said that(據說)等。

It is well known that anyone who wins prize should treat us to a good dinner.

中獎的人要請我們美餐一頓是眾所周知的。