It's not polite of you to spit in public.

有时作主语的不定式比较长,为了使句子结构显得平稳,常用形式it代替不定式放在句首,而把不定式置后,构成句型：it is +形容词 to do sth。。如果要表示不定式的动作是谁做的,一般在不定式前加for sb。如：It is useful for you to learn English well。当形容词kind,nice,wise,clever,wrong,right,foolish,careful,forget 等作表语时,不定式前常加of sb。,表示对人做某事的赞许、感激或责备等含义。如：It is very kind of you to help me。