The use of adverbs in english

I just realized that I need to get help with understanding how to use adverbs in English. Let's say that a guy is nice, and he performs every action in that way.

Does he:
a) act nice
b) act nicely

Which one is the correct one and why is it so? Don't be afraid to use grammatical terms.

>>10
AY BRUH U GOT DAT ROCK? I BE WHIPPIN SHIT HARD YO GETTIN DAT MONEY FO DEM BISHES AND DAT KUSH YO

It's simplified compared to the English you would use to write your term paper.

>>9
Why learn Bitanic or Aussish? Erryone speaks Merkinish