前者 for 是对于 的意思,it's good for you to stop smoking.戒烟是为你好.
后者 of 是属性,it's very nice of you to help me.=you're very nice to help me.
能转换成sb is/are +形容词的 就是 It is+形容词+of sb to do sth
for是是对于这件事情,修饰的是to stop smoking is good
of是修饰的人.=you're very nice to help me,总不能说help me is
nice吧+to do呢to do 好像没啥关系，就是有那么个意思去做某事对。。来说是。。。的