What can a prepositional phrase modify adverbially?

I wonder what prepositional phrase can modify when it as an adverb phrase?
I've learned about adjective + preposition these day, and I got confused.
see - It's very generous of you to bring me a present. 
( I know to bring me a present is an adverb tell us why it's very generous. But how about "of you" ?  )

"It" is a precursory subject or mere indicator that the real subject is at the end: to bring me a present. You could also say: To bring me a present | is generous of you.
Adjectives can have a complement.   Link You have to fill in " What is an adjective complement?" in the search field.