It's+careful(clever,kind,good,right,wrong等)+of sb.+to do sth. It's +difficult(easy,hard ,important,interesting等)+for sb.+to do sth.例如: It's kind of you to help me. It's hard for you to learn English well. 当形容词是表示人的性格特点和品质方面的词就常常用of sb,除此之外一般都用for sb.形式。