Plural nouns
English plurals can get weird (mouse → mice, foot → feet). Spanish is much simpler: either -s or -es. The article and any adjectives become plural too.
Word ends in a vowel → +s
casa → casas (houses). gato → gatos (cats). amigo → amigos (friends). chica → chicas (girls). libro → libros (books). The articles too: el → los, la → las, un → unos, una → unas.
Word ends in a consonant → +es
pan → panes (loaves of bread). señor → señores (gentlemen). flor → flores (flowers). ciudad → ciudades (cities). reloj → relojes (clocks/watches). The 'e' is added to make the word easier to pronounce.
Spelling traps
Words ending in -z change z to c in the plural: lápiz → lápices (pencils), luz → luces (lights). Words ending in -ión drop the accent mark: canción → canciones, lección → lecciones — because in the plural the stress naturally falls on the right syllable already.
Adjectives go plural too
Vowel ending → +s: bonito → bonitos / bonita → bonitas. Consonant ending → +es: amable → amables (when ending in -e, only -s is added); joven → jóvenes (young — accent mark appears).
