It’s + adj.+ (of / for) sb to do sth.译作“(某人)做某事是…的”.It 是形式主语,不定式结构是真正主语.of / for后面的sb是不定式的逻辑主语.
1.当形容词是可修饰人的形容词,即sb和adj.有主系表关系时,sb前用of.如 It’s clever of you to answer the question so quickly./ It’s kind of you to help me a lot.这样的形容词有：kind,nice,rude,clever,silly,foolish等.
2.当形容词是不可以修饰人的形容词,即sb和adj.没有主系表关系时,sb前用for.如 It’s easy for her to clean the window by herself./ It’s important for us to learn English well.这样的形容词有：easy,hard,difficult,important,necessary,dangerous等.