It 代替不定式短语时：(像楼上的一种形式)
1.He feels it his duty to help others.
(他觉得帮助他人是他的责任.)
2.The new medicine makes it possible to treat this terrible disease.
(这种新药使治疗这种可怕疾病成为可能.)
It 代替动名词短语时：
1.We thought it no use doing that.
(我们认为那样做无济于事.)
2.Our teacher thinks it no good learning without practice.
(我们的老师认识光学不实践是没有好处的.)
