It is + adj.+ for sb.+ to do sth.可以换成 It is + adj.+that 从句
eg.I t is very important for us to keep healthy.
相当于 It is very imprtant that we should keep healthy.
of 和 for要根据形容词的特征来区别.如果形容词是形容人的品质性格要用of；如果是表达客观事物的特征就用for.
It is very nice of you to help me with my English.