关于it 做形式主语句子的几个问题

关于it 做形式主语句子的几个问题
（1）为什么有的时候是It be adj.for sb.to do sth.有时候却用It be adj.of sb.to do sth.差别在哪里?比如：
It is very difficult for me to do it.和
It was very kind of him to pay the bill.
（2）再看下这个句子:
You ought to have locked the door.That was very careless.
变成it引导的形式主语句子：It was very careless of you not to have locked the door.为什么不改成to have not locked the door?

1.It be adj.for sb.to do sth中 adj 修饰的是sth,也就是说,adj说的是to do sth的特性
而It be adj.of sb.to do sth中 adj 修饰的是 sb,也就是说,adj说的是sb的特性
如：it is difficult for him to do the job.事实上是job 很difficult.
而 it is kind of you to do it for me.事实上是you 很kind.
2.要否定一个不定式,就要把not加在不定式之前,而不是把not就在不定式表示的行为动作上,这是个规则.