1、 作主语 不定式作主语时，谓语用单数。往往用it 作形式主语，把不定式放在谓语后面。如：
It took us two hours to finish the job.
It is impossible for us to get there on time.
It is very kind of you to help us.
注意：（1）其他系动词如look, appear等也可用于此句型。
（2）当不定式作主语的句子中又有一个不定式作表语时，不能用It is …to…的句型。试比较：
It is to negate my own idea to believe him.（错）
To believe him is to negate my own idea .（对）
