Spanish nouns follow some patterns when they become plural. However, there are exceptions, especially when it comes to foreign words.

GENERAL RULES AND THEIR EXCEPTIONS

Spanish nouns become plural by adding -s or -es to the singular form.

1. Nouns that end in an unstressed vowel or a stressed -a, -e, or –o become plural by adding -s.

mesa → mesas

sofá → sofás

pozo → pozos  

2. Nouns that end in a stressed -i or -u become plural by adding -es.

The plural form with only -s is acceptable, but adding -es is recommended, especially in formal speech.

esquí → esquíes (or esquíes)

tabú → tabúes (or tabús)

Exception

Some foreign or colloquial words that end in a stressed -i or -u become plural by adding only -s.

pirulí → pirulís

champú → champús

3. Nouns that end in a consonant generally become plural by adding -es.

reloj → relojes

rey → reyes

*At the end of any word, -y is pronounced as a vowel, yet still follows this rule and adds -es to become plural. Then, when followed by -es, the -y is pronounced as a consonant.

Remember, if the singular form ends in -z, then change -z to -c before adding -es.

avestruz → avestruces

lápiz → lápices

Exception 1

Foreign words that end in a consonant often become plural by adding only -s, especially if the consonant is not a common ending consonant in Spanish* or is preceded by another consonant.

*The most common ending consonants in Spanish are -d, -n, -l, -r, -s and -z.

chat → chats

iceberg → icebergs

When the ending letter is -y preceded by a vowel, it becomes -i.

jersey → jerséis

espray → espráis

Exception 2

For nouns that end in -s preceded by an unstressed vowel, the singular and plural form are the same.

atlas → atlas

oasis → oasis

This group includes the names of the days of the week that end in -s:

lunes, martes, miércoles, jueves, viernes

It also includes compound nouns formed by a noun already in its plural form:

espantapájaros, paraguas, lavaplatos

ACCENTUATION OF PLURAL NOUNS

The stressed vowel in the plural form of a noun is the same one as in the singular form.

(stressed syllables are underlined)

sofá → sofás

canal → canales

cama → camas

lápiz → lápices

pájaro → pájaros

sílaba → sílabas

There are three exceptions:

carácter → caracteres

espécimen → especímenes

régimen → regímenes

In some cases, to preserve the position of the stress, and due to the change in the number of syllables when adding -es, the plural form needs a written accent that was unnecessary in the singular form, or else loses the written accent that was necessary in the singular form:

imagen → imágenes

camión → camiones

LEARN THEM!

I think these are the most important points for how to make Spanish nouns and adjectives plural. Although anybody could naturally learn how to make plural nouns and adjectives by reading and listening, I recommend that all beginner students learn these rules and exceptions.