1.It is adj.+ for +sb.+ to do
与事物有关的形容词时用 for (e.g.possible,impossible,
important,necessary,essential,convenient,difficult,hard,
easy,useless...)
例句：It is difficult for him to do so.
2.It is adj.+ of +sb.+ to do
与人的赞美和批评有关的形容词时用of(e.g.nice,kind,wise,
good,polite,impolite,right,clever,bad,wrong,cruel,
stupid,foolish...)
例句：It is very considerate of you to help me.