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"Unos" is a form of "un", an indefinite article which is often translated as "a". Learn more about "un" below.
un(
oon
)An indefinite article is a word that indicates that a noun is not specific (e.g., a girl).
1. (singular)
b. an (before vowel sound)
Tienen una vieja cámara que todavía funciona.They have an old camera which is still working.