In Spanish, nouns can have two numbers: singular and plural.
When nouns end with a vocal (-a, -e, -i, -o, -u), you add -s to form the plural.
Los gatos (the cats)
Los niños (the children)
Los hermanos (the brothers)
Las sillas (the chairs)
Las niñas (the girls)
Las hermanas (the sisters)
When singular nouns end with a consonant, you add -es to form the plural.
Los profesores (the professors)
Los directores (the directors)
Los señores (the men)
Las mujeres (the women)
Las canciones (the songs)
Las universidades (the universities)
When nouns end in -í (tonic), you add -es to form the plural.
El esquí (the ski) – Los esquíes (the skis)
El jabalí (the boar) – Los jabalíes (the boars)
When singular nouns end in -z, you add -ces to form the plural.
La luz (the light) – Las luces (the lights)
La actriz (the actress) – Las actrices (the actresses)
El lápiz (the pen) – Los lápices (the pens)
El pez (the fish) – Los peces (the fish)
When singular nouns finish in -y, you add -es to form the plural.
El rey (the king) – Los reyes (the kings)
La ley (the law) – Las leyes (the laws)
Exception: when the word is foreign and ending with -y, you add -is to form the plural.
El jersey (the sweater) – Los jerséis (the sweaters)
In Spanish, there are words which can only be used in the singular form.
El norte (the north)
El sur (the south)
El este (the east)
El oeste (the west)
There are also words which can only be used in the plural form.
Las gafas (the glasses)
Las tijeras (the scissors)
Los pantalones (the trousers)
Los zapatos (the shoes)