在此动词不定式作主语,用it作形式主语 .
在很多情况下,人们通常用it作为形式上的主语,而把不定式移到谓语之后,使句子结构显得平稳一些.例如：
It's rude to turn your back to your teacher and refuse to answer.背对着老师,拒绝回答问题是不礼貌的.
It is impossible to learn a foreign language without making painstaking effort.想不下苦功就能学会外语是不可能的.
It seemed a pity to have wasted so much time.浪费了这么多时间,真是遗憾.