itisadjforsbtodosth.doc

It is +adj+for sb to do sth与It is+adj+of sb to do sth  
It is+adj.+of sb. +to do sth中的adj.跟sb.有关,
这个adj.是用来形容sb.的,
表示某人真是太adj.了．

而It is +adj.+for sb.+to do sth表示做某事对某人来说很adj.
这里的adj.是修饰do sth.的 
表示做某事真是太adj.了
It’s very kind of/for you to help me. 
It’s clever of/for you to work out the maths problem.
It’s very dangerous for/of children to cross the busy street. 
It’s difficult of/for us to finish the work.   
It is careless of/for him to lose so many things.
It’s very important for/of you to practise your English.
It is necessary for/of you to learn from others.
It is impossible for/of you to get there in such a short time.
It is impolite of/for you to play tricks on your teacher.
It is possible for/of me to spend one hour finishing the job.
It is important for/of you to study hard. 　

It is wise of them to turn down the suggestion.
他们很明智，拒绝了这个建议。
It is wise for them to turn down the suggestion.
他们拒绝这个建议是明智的。
前句强调them是wise的；后句强调to turn down the suggestion 是wise的。