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"Un" is an indefinite article which is often translated as "a", and "tres" is an adjective which is often translated as "three". Learn more about the difference between "un" and "tres" 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.
tres(
trehs
)An adjective is a word that describes a noun (e.g., the big dog).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
2. (number)
a. three 
El tres es mi número de la suerte cuando juego en los casinos.Three is my lucky number when I gamble in casinos.
b. third  (in dates)
Hoy estamos a tres de diciembre. ¡Cómo pasa el tiempo!Today is the third of December. Time sure does fly!
a. tres 
El tres es un instrumento muy asociado con el son cubano.The tres is an instrument greatly associated with Cuban son.