Definition: The plural form of a noun indicates that the quantity of the described by that noun object is greater than one.
In Spanish, the plural form of a noun is expressed by a plural indefinite or definite article - unos/los for masculine and unas/los for feminine nouns - and, generally, a plural ending -s or -es.
Example:
masculine plural nouns: unos libros (some books), los arboles (the trees)
feminine plural nouns: unas palabras (some words), las camas (the beds)
Below are the general rules for the formation of plural nouns.
Nouns ending with a vowel -a, -á, -e, -é, -i, -o, -ó, -u take the ending -s when forming plural:
la película → las películas (the movies)
el taxi → los taxis (the taxis)
el baño → los baños (the bathrooms)
Exception:
el no → los noes (the noes)
el yo → los yoes (the "I"s/ egos)
Nouns ending with the stressed vowel -í or -ú, the endings -es and -s are used interchangeably:
el ají → los ajíes or los ajís (the chilis)
el tabú → los tabúes or los tabús (the taboos)
Exception:
el sí → los síes (the yeses)
However, words of foreign origin that end with -í or -ú typically add the -s for plural:
el popurrí → los popurrís (the potpourris)
el menú → los menús (the menus)
Nouns ending with a consonant -d, -j, -l, -n, -r take the ending -es when forming plural:
la pared → las paredes (the walls)
el tren → los trenes (the train)
el reloj → los relojes (the clocks)
However, foreign words that end in a consonant typically add the -s for plural:
el iceberg → los icebergs (the icebergs)
el tuit → los tuits (the Tweets)
Nouns ending with -y take the ending -es when forming plural:
la ley → las leyes (the laws)
el rey → los reyes (the kings)
Interesting fact: The Spanish 'y' may act as a vowel (when at the end of the word) and as a consonant (when it starts off a syllable).
However, foreign words that end in -y take the plural ending -s and may change -y to an -i:
el jersey → los jerséis (the jerseys)
el gay → los gays or los gais (the gays)
Nouns ending with -c or -g may change to qu and gu respectively when taking the ending -es to form a plural. Or they may simply add the ending -s:
el frac → los fraques or los fracs (the fraques)
el zigzag → los zigzagues or los zigzags (the zigzags)
Nouns ending with a -z change z to a c when taking the ending -es to form plural:
la luz → las luces (the lights)
el lápiz → los lápices (the pencils)
Nouns ending with -s or -x do not add a plural ending if the last syllable is unstressed:
la crisis → las crisis (the crises)
el viernes → los viernes (the Fridays)
el tórax → los tórax (the thoraxes)
Yet those nouns add the plural ending -es if the last syllable is stressed (or it is a monosyllable noun):
el autobús → los autobuses (the buses)
el país → los países (the countries)
el mes → los meses (the months)
Nouns that have an accent mark on the last syllable drop that accent mark when forming plural:
el corazón → los corazones (the hearts)
la conversación → las conversaciones (the conversations)
Some nouns gain an accent mark when forming plural:
el examen → los exámenes (the exams)
el joven → los jóvenes (the young people)
Note: The addition of an accent mark is done to preserve the stress on the correct syllable in a word.
Compound nouns that are formed with the verb+plural noun tend to have the same singular and plural forms:
el/los paraguas - the umbrella/-s
el/los abrecartas - the letter opener/-s
el/los abrelatas - the can opener/-s
Some Spanish nouns exist only in plural form:
los víveres - the supplies
los celos - the jealousy
las fauces - the jaws
When a group of objects contains at least one object of masculine gender, it takes the masculine plural form regardless of the count of feminine objects. The meaning of the word is then neutralized:
1 gato + 100 gatas = 101 gatos (1 male cat + 100 female cats = 101 cats)