Unlike ex2, when a judgemental adjective for the subject is used, it's followed by "of" not "for". Do you know why "of" should be used? I guess it's because these adjectives are properties of a person, so "of" can mean some possessive property. Can someone explain the reason?
ex1)It was nice(stupid, foolish, smart) of him to help the old lady.
ex2)<=> It is important for us to learn English.
Not a teacher.
Of can also be used "possessively" to indicate origin. E.g. John, C.E.O. of GM. (GM's C.E.O., John.)
"Nice" is a characetistic of "him"
"Important" is a characteristic of "learning English", not of us. It's important for us; but not of us.
If someone said, "How important of you, learning English!" - it could be a sarcastic statement indicating that you think you are important because you are learning English.