it用来充当形式主语或形式宾语，后面的真正主语或真正宾语通常是不定式结构、动名词或名词性分句。It的这种用法叫作先行it。
eg.
It is quite important for us to form a good studying habit.
I think it important to finish the task in time.
it用来充当形式主语，后置的真正主语往往可以取代先行it的位置，出现在句首。
Eg. To form a good studying habit is quite important for us
并非所有先行it结构都可转化，下边句子只能作如下转化。
eg.
It seems that John is not coming after all.
John doesnt seem to be coming after all.
Cleft it :
Task
1. 句子的基本结构
2 .分裂句的本质
3 . 分裂句的结构
4 .假拟分裂句
句子结构
主语+谓语
已知信息
新信息
末端中心
末端重心
Eg. I told him the news yesterday.
分裂句的本质
Eg. I told him the news yesterday.
Eg. It is I that told him the news yesterday
分裂it
分裂句是以it为引导词的强调句型。它的结构形式是：It +be 的一定形式+中心成分+that /who分句。通过这种结构可以强调除谓语动词以外的大多数句子成分。说话人通过分裂句分别强调主语、间接宾语、直接宾语、状语，使之成为信息中心。分裂句中的谓语可以采取复杂形式
Eg.
It is I who am to blame.
It may have been at Christmas that John gave Mary a handbag.
It was in Shanghai that I got my Masters Degree.
主语补语通常不作分裂句的中心成分。宾语补语可以这样用。
It is beautiful that she is.
It is Chairman of the committee that they elected him
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