Adjectives and Adverbs | Grammar Rules
https://www.grammarbook.com/grammar/adjAdv.aspRule 1. Many adverbs end in -ly, but many do not.Generally, if a word can have -ly added to its adjective form, place it there to form an adverb.. Examples: She thinks quick/quickly. How does she think? Quickly. She is a quick/quickly thinker. Quick is an adjective describing thinker, so no -ly is attached. She thinks fast/fastly. Fast answers the question how, so it is an adverb.