un/una/unos/unas

In Spanish, an indefinite article is a word that comes before a noun without presenting its referent as already identifiable to the listener. They are the equivalent of 'a,' 'an,' or 'some' in English.

Think of the difference between:

• Veo un coche. (I see a car.) - A car introduced without identifying it for the listener; the speaker can be looking at a particular car.
• Veo el coche. (I see the car.) - A specific car that we both know about.

Spanish has four indefinite articles, and the one you use depends on the gender and number of the noun it describes:

• un: for singular, masculine nouns (e.g., un libro - a book)
• una: for singular, feminine nouns (e.g., una casa - a house)
• unos: for plural, masculine nouns (e.g., unos libros - some books)
• unas: for plural, feminine nouns (e.g., unas casas - some houses)

To use these correctly, you need to know the gender and number of Spanish nouns. This principle of agreement is also central to adjective agreement.

Feminine Nouns Beginning With Stressed /a/
Una normally shortens to un immediately before these nouns: un águila blanca. Águila remains feminine, so blanca agrees in the feminine. If a word intervenes, use una: una gran águila. Full una águila is also possible, though uncommon.

When There Is No Article
Do not copy every English a: a plain profession after ser usually has no article, as in Soy profesora. A modifier can allow an article: Es una profesora excelente. Plural and mass nouns can also appear without an indefinite article: Necesito bolígrafos; Quiero agua. Unos/unas can present an unspecified group; they are not compulsory before every plural noun.