it是人称代名词，在句中用作简单的指代、非人称的指代、形式上的主语或宾语等。

用作简单代名词的it?当句子中的有些成分重复出现时，可以用IT代替前面出现过的名词或其同等语、其他片语、子句或句子。

1. it代替前面出现过的名词或其同等语。

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

我上周买了一辆新车，但它昨天就被盗了。

it代替名词片语a new car。

2. it代替前面出现过的其他片语。

I tried to calm him, but it was impossible.

我试图使他平静下来，但这不可能。

it代替不定词片语to calm him。

3. it代替前面出现过的子句或句子，但用在动词forget, know, remember, try等的后面时通常省略。

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

你的彩票中了头奖。这会给你带来很多钱。

It代替句子Your lottery wins the first prize。

用作先行代名词的it

用作先行代名词的it主要充当形式上的主语或宾语。

1. it作形式上的主语或宾语时，后面真正的主语或宾语通常是不定词片语、动名词片语或名词子句。

It is true that he won the top prize.

他得了大奖是真的。

名词子句that he won the top prize是真正主语。

2. it在"It seems (appears, looks, chances, happens) + that (as if)..."的结构中作主语时，没有具体意义。

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

似乎应该让他请客。

3. it作形式主语时后面可以接is+过去分词+that引导的名词子句。如：

It is believed that(据信)

It is reported that(据报导)

It is well known that(众所周知)

It is said that(据说)等。

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

中奖的人要请我们美餐一顿是众所周知的。