Its very kind you to say so

1.It's necessary ___for___us to master a language.
2.It's unwise ___of___them to refuse our help.
3.It is very important ___for__us to study some rules.
4.It is unpracticable ___for___us to finish such a difficult task
in such short time.
5.It's wrong __of / for___a person to think only of himself. （这一句for / of 都可以,看你站在什么立场上,出题人不太严谨）
窍门：
如果被认为是普遍现象,就用for,如：
1.It's necessary ___for___us / people to master a language.
2 It is very important ___for__us / learners to study some rules.
3 .It is unpracticable ___for___us / anyone to finish such a difficult task in two days.
如果被认为是某个人的个别现象,我们就用of,如：
1 It was wrong of me / John / us / the girl to be late again ans again.
2 It is very kind of him to give us so much help. (也就是说,he肯,但别人不一定肯)

应该是用for的 用of有点别扭

of
到底用for还是of有一个小窍门，能改成sb is kind to do 的用of,不能的用for
如：本句可改成 You are very kind to say so.
It's hard for you to speak French. 是speak French很难，不是you are hard

一楼除了第一题以外都对，不能只说what hard work，要 what a hard举个例子吧、：It is not very hard for you to find English is