How Arabic says quickly, suddenly, openly — and why they look like nouns.
Arabic has few dedicated adverbs. Instead it puts a noun in the accusative case — marked by the ending ـًا (tanwīn fatḥ) — and that noun then describes how the action happened. This is the ḥāl construction.
So كَثِير is the adjective "much", while كَثِيرًا is "much / often" describing a verb. The ـًا ending is the whole difference.
These are nouns too, and many double as prepositions when a noun follows.