Some Spanish adjectives are used as adverbs with certain verbs. These are known as adverbios adjetivales and can be recognised by their -o ending, which never changes – no matter whether the noun is masculine or feminine, singular or plural. For example, "Las chicas trabajan duro - The girls work hard" - Duro is an adverb as indicates HOW they are working, not how they like. A trick to check if an adjective is used as an adverb is to add the suffix -mente to it (the way a lot of adverbs end). In the example, if we add the suffix -mente to duro "duramente", the sentence still makes sense = it's an adverb. "Las chicas trabajan duramente - The girls work hard".