Thanks Darby:)

The main difference is that "quickly" and "rapidly" are adverbs, and are used to describe verbs. Ex: We need to move quickly if we're going to catch the bus. The train rapidly approached the station. "Rapidly" is slightly more formal, but they mean the same thing. The adjective forms of both words are "quick" and "rapid" and can be used to describe nouns. "Fast" is both an adverb AND an adjective, so it can describe verbs AND nouns. Ex: I like fast cars. He ran fast so he could make it to school on time. I hope this helps!