Plurals In Spanish

If a noun ends in a vowel, make it plural by adding "s".

libro book libros books
mesa table mesas tables

If a noun ends in a consonant, make it plural by adding "es".

ciudad city ciudades cities
borrador eraser borradores erasers
words that end in "y" use the above rule
ley law leyes laws

If a noun ends in "ión" add "es" and drop the written accent.

avión airplane aviones airplanes
televisión television televisiones televisions

If a noun ends in "z", change the "z" to "c" and add "es".

lápiz pencil lapices pencils
actriz actress actrices actresses

If the plural refers to a mixed group (both masculine and feminine nouns) use the masculine plural form.

dog perro or perra dogs los perros
parent padre or madre parents los padres

The days of the week for Monday - Friday do not change in the plural.  The days for Saturday and Sunday do change in the plural.

el lunes Monday los lunes Mondays
domingo Sunday los domingos Sundays

If an adjective that ends in "o" modifies a plural noun it must also be made plural.

el apartamento pequeño los apartamentos pequenos
the small apartment the small apartments

If an adjective ends in "o" and it modifies a feminine noun, drop the "o" and replace it with "a".  This must be made plural if the noun is also plural.

la acoba pequeña las alcobas pequeñas
the small bedroom the small bedrooms