it is...for sb.to do...和it is...of sb.to do...的用法

给您介绍一个最简单的辨别方法：如果be动词后的形容词是说明不定式的逻辑主语,就用of sb.to do...；如果是说明整个不定式的,则用 for sb.to do...；例如：
1、It is very kind of you to help us.（可以转换成：You are very kind to help us.kind
说明 you 的属性）；
2、It is possible for us to get there before dark.(不可转换成：we are possible...,而应该是 for us to get there is possible)

for限制于 of属于
如果是人的自然属性，就用of，比如kind,cruel,stupid,clever,nice,bad,
如果不是，就用for,比如important,necessary,hard,easy,