It's for sb..和 It's of sb..
1）for sb.. 常用于表示事物的特征特点,表示客观形式的形容词,如easy, hard, difficult, interesting, impossible等：
It's very hard for him to study two languages. 对他来说学两门外语是很难的.
2）of sb.的句型一般用表示人物的性格,品德,表示主观感情或态度的形容词,如good, kind, nice, clever, foolish, right.
It's very nice of you to help me. 你来帮助我,你真是太好了.
for 与of 的辨别方法：
用介词后面的代词作主语,用介词前边的形容词作表语,造个句子.如果道理上通顺用of,不通则用for.如：
You are nice. (通顺,所以应用of).
He is hard. (人是困难的,不通,因此应用for.)