In Spanish, these are the ways to form the plural:
-when substantives end with a vocal (-a, -e, -i, -o, -u), you add an ‘s’ to form the plural.
ex. Los gatos (The cats)
-when singular substantives end with a consonant, you add ‘-es’.
ex. Las mujeres (The women)
-when singular substantives end in -í (tonic), you add ‘-es’.
ex. Los esquíes (The skis)
-when singular substantives end in -z, you add ‘-ces’.
ex. Las actrices (The actresses)
-when singular substantives end in -y, you add ‘-es’.
ex. Las leyes (The laws)
Words that have more than two syllables and end in -as, -es, -is, -os, or -us don’t change in the plural form.
ex. El paraguas -> Los paraguas (The umbrella)
El lunes -> Los lunes (Monday)
When they end in -és you add -es:
ex. Francés -> franceses (French)
Inglés -> ingleses (English)
The name of vocals and consonants:
Vocal + -es: a -> aes
Consonant + -s: be -> bes
-Plural of compound words
In Spanish, compound words can be:
a sentence with a preposition: bolso de mano (handbag)
two substantives: impresora láser (laser printer)
a substantive + adjective: cajero automático (cash machine)
When you have a sentence with a preposition you only turn to plural the principal noun:
ex. Bolso de mano (handbag) -> bolsos de mano
When you have two substantives you only write the plural of the first one:
ex. Impresora láser (laser printer) -> impresoras láser
When you have a substantive followed by an adjective, you turn to the plural form both the substantive and the adjective:
ex. Cajero automático (cash machine) -> cajeros automáticos
-Plural of symbols and acronyms
Symbols are graphic representations of scientific or technical concepts.
In Spanish, symbols never have a plural form. ex. 3 horas = 3 h, NOT 3 hs.
You never write plural acronyms, in Spanish. You use articles to know if it’s singular or plural.
ex. ONG = Las ONG, NOT ‘ONG’s’ or ‘ONGS’.
OTAN = La OTAN (NATO)