This is regarding "rapidamente" (an adverb) that modifies a verb or action. "Rapido" is an adjective

[Ellis_P]

that modifies a noun. I used "rapidamente" to modify the verb of habla. "Rapido" is used only with a noun. I learned this from a good grammarian.
Thank you. Any objection?

[OnesimusUnbound]

None at all. To generalize, adjectives modify noun while adverbs modify verb, adjective and other adverb.