It is really nice (of) you,it is important (for) you to do that.请接合这句话,讲解一下(of)和(for)的用法

It is really nice (of) you,等于You are really nice .
it is important (for) you to do that.此句不可以说You are important to do that.
当形容词能直接修饰或说明人的时候就用of
当形容词不能直接修饰或说明人的时候就用for