Adjectiveadverb

Adjectiveadverb is a term used in some linguistic descriptions to refer to words that serve as both adjectives and adverbs, or to adjectives used in adverbial function without the typical -ly suffix. In English, adverbs most often end in -ly (quick → quickly). But many adjectives can occur in adverbial position without changing form, producing what is sometimes called a flat adverb: drive slow, speak loud, work hard. These uses are common in informal speech and some dialects but are considered nonstandard in formal writing because they do not provide the -ly morphology typically associated with adverbs.

Some adjectives have stable adverbial equivalents that share the same form as the adjective (fast, hard). For instance, "The car goes fast" uses "fast" as an adverb. In "The car is fast," "fast" is an adjective. The boundary between adjective and adverb is thus blurred in such cases.

Cross-linguistic variation: in many languages there is no distinct adverbial suffix, and adjectives or other word classes modify verbs in ways that overlap with adjectives. In English, the standard remedy is to use -ly adverbs; some adjectives pair with adverbs via separate forms (quickly, slowly). The term adjectiveadverb is more of a descriptive label used by some grammarians rather than a widely recognized category in modern grammars.

Notes: the use of flat adverbs tends to be more acceptable in informal registers and some varieties of American English; in formal contexts, writers tend to prefer the standard -ly adverbs.