fun as a noun:
I had fun at the carnival.

fun as an adjective:
I had a fun time at the carnival.
She had a more fun time at the carnival than me.
He had the most fun time of all of us.
She had a funner time at the carnival than me. [slang]
He had the  funnest time of all of us. [slang]

fun as a verb:
He is just funning you.

Grammar Girl has a more detailed explanation.