It is+adj.+of sb. 这里的形容词是形容人的, 你可以假设人是---的,若通顺就用of eg: It is kind of you to help me. (you are kind.) It is friendly of them to lend money to me. (they are friendly.) It is+adj.+for sb. 这里的形容词是修饰后面的物或动作的。与人没关系,你假设人是---的,不通顺就用for 。 It is hard for him to carry the heavy box. (He is hard.)
看那个形容词. 当那个形容词是形容人的品格,人的特征时,用of;例:It's very kind of you to help me. 当那个形容词是形容做的事的特点时,用for.例:It's very difficult for you to finish much homework in one hour.