指代时间、距离、自然现象等
例句：
It’s a lovely day isn’t it?
It was very cold today.
It’s a long way to go.
What time is it ? It’ s half past eleven.
指代前面所提到的人或事物，或指代说话双方都熟悉的人、婴儿等。
例句：
Who is knocking at the door?
It’s me.
Sorry I am late again . It doesn’t matter.
Thank you. Don’t mention it.
3) 作形式主语或形式宾语，而把真正的主语或宾语放在后面，真正的主语或宾语往往是从句，动名词、不定式等。例如：
例句：
He found it not easy to learn a foreign language well.
We think it no good reading in bed.
I think it necessary that we have a meeting.
It is said that all of them have gone to the cinema.
It is no use trying to persuade him.
It is well-known that China has the largest population in the world.
4) it 用于虚指，尤其是表示喜好、憎恶的动词，如;enjoy, like, love, dislike, hate,
see to后，即这些动词后面的宾语从句前要加 it.
例句：
I hate it when people speak with their mouth full.
I will appreciate it if you can help me.
Please see to it that you bring enough money when you go out.
练习：
1)Do you like _______ here? Oh ,yes. The air, the weather, the way of life. Everything is so nice.
A. this
B. these
C. that
D. it
2) I prefer a flat in Inverness to ______ in Perth, because I want to live near my Mom’s.
A. one
B. that
C. it
D. this
3)______ is our belief that improvements in health care will lead to a stronger, more prosperous economy.
A. As
B. That
C. This
D. It（浙江）
4) Cars do cause us some health problems___ in fact far more serious ______ than mobile phones do.
A. one
B. ones
C. it
D. those