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.