不定式或不定式短语作主语时,常用it作形式主语,避免头重脚轻,这时大多构成“It is + 形容词(+for sb.)不定式”结构.
It is impossible for him to give up smoking.
但也可能用于其它句型,如：
It took Professor Chomsky near a decade to finish writing this book.
值得注意的是：
在It is adj for sb to do sth 句型中,如果做
表语的形容词是表现人的内在的性质特点的,如：honest,cruel,warm-hearted,stupid,nice,kind,careful,considerate,selfish,mean(小气的）等,需用of sb to do sth.如：
It is very kind of you to have come all the way to the airport to see me off.
It is very careless of you to have made so many mistakes in this exam.