For some, distinguishing adjectives from adverbs is impossibly confusing. Yet telling them apart should be easy. Adjectives modify nouns or pronouns (tight shoes, She is brilliant!), while adverbs modify verbs, adjectives, and other adverbs (drive carefully, rather hasty, more rapidly). Adjectives provide answers to “what kind,” “which one,” and “how many,” while adverbs answer “how,” “when,” and “where” (to boldly go, see you later, happening here).
Simply put, adverbs modify everything that adjectives don’t—including whole sentences! They are a grammatical wastebasket—the part of speech into which you toss anything you can’t otherwise categorize.
The source of bewilderment, then, may not be function but form. We think of adverbs as typically ending in -ly (badly, quickly, completely), unlike their adjective counterparts (bad, quick, complete). But some adjectives end in -ly (cowardly lion, motherly affection, friendly persuasion), while some adverbs, called “flat” adverbs, do not (sit up straight, work hard, aim high). To add to the ambiguity, a small number of words can function as adverbs with or without the classic ending (walk slow on the ice / speak more slowly; hold me close / a closely knit family). Still others shift meaning as they change form (She arrived late. Lately, she’s been doing that). And some are both adjectival and adverbial without changing form (fast trains, run fast; early morning, wake up early). No wonder the mind boggles.
Perhaps in response, there has been a resurgence of common adjectives used adverbially (You played amazing. It worked out fantastic.) Similar flat adverbs, like sudden, extreme, and wondrous, were standard in early Modern English. But in the 18th century, grammar mavens began to disparage them, insisting on the -ly form, and for certain adverbs, that is now the norm. While our language may be shifting back toward increasing use of flat adverbs, an adjective where an adverb is expected may still be subject to criticism. It’s fine to use these newly flattened adverbs with friends, on social media, etc. But traditional cautions apply. It’s probably best to stay with established forms in academic writing, during a job interview, and in other circumstances that call for more formal language. You’re bound to do “great”!