Un, Una, Unos, Unas

The indefinite article introduces something new or not specific, like English a/an and some. It agrees with the noun in gender and number:

• un: masculine singular, un perro (a dog)
• una: feminine singular, una casa (a house)
• unos: masculine plural, unos niños (some children)
• unas: feminine plural, unas flores (some flowers)

Use it after hay (Hay un banco aquí), when you mention something for the first time (Tengo una pregunta) and with ser + noun + adjective (Es un chico muy simpático). A profession on its own takes no article: Soy profesora, but Es una profesora muy buena. The plural unos/unas means some or a few: unos días (a few days), unas gafas (a pair of glasses). Watch nouns whose gender does not follow -o/-a: un problema, un día, una moto.