Adjective or adverb? 1

Adjectives describe nouns — An adjective modifies a noun and must agree with it in gender and number: Es una chica inteligente. It stays the same regardless of the verb used — including after ser, estar, parecer, quedarse: Está cansada, not cansadamente.

Adverbs modify verbs and adjectives — An adverb describes how an action is done or how strong a quality is. It never changes form. Most Spanish adverbs are formed by adding -mente to the feminine adjective: lenta → lentamente, fácil → fácilmente. They can go before or after the verb but never between article and noun.

The quick test — Ask yourself: does the word describe a thing (noun)? → adjective. Does it describe how something is done (verb/adjective)? → adverb. Habla rápido uses the adjective form as an informal adverb — but the safe choice is always rápidamente.