为了平衡句子结构,it作形式主语,位于句首,而真正的主语是动词不定式（短语）动名词或从句。
It+be+adj.+(for sb.)+to do sth.
adj.：easy,difficult,hard,necessary,(im)possible,(un)likely,right,wrong,important,legal,illegal,clear,obvious,certain,suitable,
proer,fit,useful,useless,dangerous等用于以上句型。
例It is very easy for these children to pass the final exam if they work hard.
对这些孩子们来说如果努力学习的话,通过考试是非常容易。
It + be +adj. +( of sb . ) +to do sth
kind , nice , rude , cruel , considerate ,thoughtful,thoughtless , careful
careless ,silly ,(im ) polight, foolish ,stupid , clever , wise , crazy , selfish等用于以上句型。
例lt' s kind of you to help me carry the suitcase.
(2)It作形式主语,真正的主语是后面的动词-ing形式。
It is no use doing sth.做….没用。
It is no good doing sth.做….没好处。
It is fun doing sth.做…..有趣。
It is no need doing sth.做….没必要
例lt' s no use quarreling over it.争论这件事没有用处。
例 It ' s said that the novel has been translated into over 12 languages.
It is/has been +时间段+since从句 自从……已有多长时间了。
It was not/will not be long before…..不久就……
It’s one’s turn to do sth.轮到某人做某事
It’s up to sb. to do sth.由….决定;该由…..做某事。
It takes sb. some time to do sth.某人花时间做某事。
It is high/about time to do sth./that从句 是……的时候了
It is/was the first/second….time that…..第……..次做……
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