Just like definite articles, indefinite articles indicate the gender (masculine or feminine) and number (singular or plural ) of a noun and have four forms, which are listed below.
Indefinite Article Forms
Number Gender Form
singular masculine
un
singular feminine
una
plural masculine
unos
plural feminine
unas
Matching Gender and Number
The indefinite article always has to match both the gender and number of its noun. For example if a noun is masculine and singular (like gato ), then its article also has to be masculine and singular (so, un gato ). If that same noun were plural (like gatos ), the article would also be plural (so, unos gatos ).
Here are some more examples showing indefinite articles matching the number and gender of the nouns they modify.
¿Hay un elefante en este zoológico?
Is there an elephant in this zoo?
Quiero una galleta.
I want a cookie.
Hay unos niños en el patio.
There are a few children on the patio.
Compré unas faldas nuevas ayer.
I bought some new skirts yesterday.
Exceptions
When a feminine singular noun begins with a stressed a or ha, the masculine indefinite article (un) is used instead of the feminine indefinite article (una).
When the same noun is plural, the feminine article is used.
Here are some examples of feminine nouns that take the masculine indefinite article in the singular. As the table shows, they take the feminine plural article in the plural.
Singular Plural
un águila
unas águilas
un arpa
unas arpas
un hacha
unas hachas
un aula
las aulas
