Question

It's+adj.+of sb+to do sth是什么意思？

Answer 1

表示人的性格特征如勇敢，大胆，聪明，善良，残忍，机灵等等词用于该句型。如果不是这些词句型中的of 就要用for.
IT is clever of Tom to refuse the task.
IT is bold of them to attend the big gathering.
别的词不表示人的性格特征的词 换 for
it is necessary fir us to kearn a foreign language.
it is not easy for you to do the job.

Answer 2

这是动词不定式的复合结构，请看例句：
It is very nice of you to help me with my English.
您帮我学英语，您真是太好了。