1.It's necessary ______us to master a language.
2.It's unwise ______them to refuse our help.
3.It is very important _____us to study some rules.
4.It is unpracticable ______us to finish such a difficult task
in such short time.
5.It's wrong _____a person to think only of himself.
哪些用of哪些用for，请说明原因，谢谢。我老分不清楚。
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)
答案是1.for 2.of 3.for 4.for 5.of
2. for （理由同上）
3. for （同上）
4. for （同上）
前四个题里都含有一个相同的句型：It's + adj.+ ( for sb. ) + to do sth. “对某人来说做某事是......的。”
5. for
for与of的区别：
介词for可以用来表示“原因”或“目的”；of则表示所属关系，是“名词所有格”这个语法项目里的一部分，翻译出来是“......的”，但翻译的时候要从of后往of前翻译。例如翻译“Maria是我的朋友之一”： Maria ia a friend of mine.追问
答案是1.for 2.of 3.for 4.for 5.of
追答for sb. to do sth.与of sb. to do sth.的区别：
1.如果说明不定式表示的动作是谁做的，可以在不定时前加一个for引起的短语。如：
1）It's important for us to learn English well.
2）It's not hard for one to do a bit of good.
2.以某些形容词，如kind, good, nice,clever,foolish,wrong等作表语时，不定时前场可以加一个of引起的短语，来说明不定式指的是谁的情况。例如：
1）It's very kind of you to think so much for us.
2）It's good of you to come and meet me.
3.在too和enough后面常用带for的结构。例如：
1）The box is too heavy for me to lift.
2）The sweater is large enough for her to wear.
