In Spanish, forming plurals involves changing the singular form of a noun, adjective, or article to indicate more than one item. The rules for making plurals in Spanish are generally straightforward, but they do depend on the ending of the word in its singular form. Here's a breakdown of how plurals work:

1. Plurals of Nouns

A. Nouns Ending in a Vowel (a, e, i, o, u)

• Rule: Add "-s" to the end of the word.
• Examples:
  • Libro (Book) → Libros (Books)
  • Casa (House) → Casas (Houses)
  • Amigo (Friend) → Amigos (Friends)
  • Silla (Chair) → Sillas (Chairs)

B. Nouns Ending in a Consonant

• Rule: Add "-es" to the end of the word.
• Examples:
  • Profesor (Teacher) → Profesores (Teachers)
  • Rey (King) → Reyes (Kings)
  • Ciudad (City) → Ciudades (Cities)
  • Mujer (Woman) → Mujeres (Women)

C. Nouns Ending in "-z"

• Rule: Change the "-z" to "-c" and add "-es."
• Examples:
  • Lápiz (Pencil) → Lápices (Pencils)
  • Voz (Voice) → Voces (Voices)
  • Pez (Fish) → Peces (Fishes)

D. Nouns Ending in an Unstressed "-s" or "-x"

• Rule: These nouns do not change in the plural. The article or context indicates plurality.
• Examples:
  • El lunes (The Monday) → Los lunes (The Mondays)
  • El tórax (The thorax) → Los tórax (The thoraxes)

2. Plurals of Adjectives

Adjectives must agree in gender and number with the nouns they modify. The rules for forming plurals of adjectives are similar to those for nouns.

A. Adjectives Ending in a Vowel

• Rule: Add "-s" to the end of the word.
• Examples:
  • Grande (Big) → Grandes (Big)
  • Bonito (Pretty) → Bonitos (Pretty)
  • Interesante (Interesting) → Interesantes (Interesting)

B. Adjectives Ending in a Consonant