Adverbs modify verbs, adjectives and other adverbs. Unlike adjectives and verbs, they are invariable in form, never changing in ending to agree with other elements in number, gender or person.

A large group of adverbs are derived from adjectives, adding a common ending. Others are similar or identical to the corresponding adjective.

These adverbs are formed by adding the ending -mente to the feminine form of -o¹ adjectives, and to the singular form of others². This corresponds to the English adverbial ending -ly (skillfully, rapidly, etc.) Where applicable, original written accent is maintained³.

1. claro→clara→claramente¹; rápido→rápida→rápidamente^{1, 3}
2. inteligente→inteligentemente²; leal→lealmente²

When two or more -mente adverbs occur, the ending is dropped from all but the last:

• Se terminó el proyecto rápida y exitosamente.
  • The project was completed quickly and successfully.

Not all adjectives take the -mente ending; some that do not include:

• difícil
• adjectives describing colors (negro, blanco, etc.)
• those describing physical traits (feo, gordo, etc.)
• adjectives of nationality (italiano, mexicano, etc.)

It is important to distinguish contexts in which an adverbial modifier is called for and those in which an adjective is required. Adjectives¹ show agreement, but adverbs² do not; examples:

• Joaquín y Elena tienen mucha¹ energía. Trabajan mucho².
  • Joaquín and Elena have a lot of energy. They work a lot.
• Admito que soy un alumno perezoso. Hay pocas¹ personas que estudian tan poco² como yo.
  • I admit that I'm a lazy student. There are few persons that study as little as I do.
• Estoy medio² dormido. Quiero descansar por media¹ hora.
  • I'm half asleep. I want to rest for half an hour.
• Pedro, tú comes demasiado². Consumes demasiada¹ comida.
  • Pedro, you eat too mucho. You consume too much food.
• Ellas son las mejores¹ (peores¹) jugadoras del equipo. Juegan mejor² (peor²) que yo.*
  • They are the best (worst) players on the team. They play better (worse) than I do.

  *More on irregular comparative forms.

Muy (very) functions only as an adverb; mucho can be either an adjective or an adverb (see examples above). They are not interchangeable, and are at times confused in some common contexts:

• Mucho, and not muy, is used as a modifier in weather expresssions, because these involve a following noun*, not an adjective:
  • Hace mucho calor* (frío*) aquí en agosto. (Lit., It makes much heat, cold...)
• Mucho is used rather than muy when the modified element* is omitted:
  • Pedro es muy listo*, ¿no? Sí, mucho.
    • Pedro is very smart, isn't he? Yes, very.