It 句型
Revision:
1.
Basic structure:
There + be + subject + adverbial
2.
Non-finite structure
There to be --------- for ; Verb
There being ---------in,on….; noun;                  adverbial 
There being a lawn extends from the river to the house.
There is a lawn extending from the river to the house.
There extends a lawn from the house to the river.  (?)
From the house to the river lies a lawn.
From the wall comes the voice of the doctor. 

It:
1.
Reference it:
Beijing is a big city. It is the capital of China.
2.
Non-reference it
A. Empty it
B. Anticipatory it
C. Cleft it 
It 句型包括由“非指代性it”作型式主语的三种句子：第一种是以it作形式主语的表示时间、距离、天气等意义的句子；第二种是以it作先行主语的句子；第三种是以it作引导词的分裂句。

Empty it
虚义it是一种非指代性it，以区别于人称代词it，常用作没有具体意义的主语，出现于表示气候、天气、温度、时间、地点、距离等意义的句子中。也用来表示一般的笼统的情况。也用于一些结构及习惯语中。
Eg.
It’s fine today, isn’t it.
It was dull when Mary was away.
It looks as if the college is very small.
Hop it. (Go away)