【1】形容词说的是“人”：It is + adj.+of sb.+ (not) to do o=sb is +adj. +to do的句型of sb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.

=You are very nice to help me.

你来帮助我，你真是太好了。

【2】形容词说的是“事物”It is + adj.+for sb.+ (not) to do .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, important, 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。

It is so kind of sb to do sth 

完全可以转成 sb 作主语，看下面句子

=It is so kind of Tom to work out the math problem for me.

可以理解为 Tom is so kind to work out the math problem for me(不定式作状语)

解答如下：

你的理解是正确的，不定式作状语