用of的话,句中的形容词描述的是人的品质,如nice,kind,polite等,很明显这些形容词主语直接就是某人~
用for的话,句中形容词指的是一件事情的性质,如important,easy,difficult等来描述一件事情,真正的主语是后面的动词不定式,it是形式主语~
所以,上述句子的答案为：It is very nice of you to offer me a seat.
It is very important for you to practice English.
It is difficuclt for you to finish the work.
It is impolite of you to fool your teacher.