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"Entertaining" is a form of "entertaining", an adjective which is often translated as "entretenido". "An" is an indefinite article which is often translated as "un". Learn more about the difference between "an" and "entertaining" below.
an(
ahn
)An indefinite article is a word that indicates that a noun is not specific (e.g., a girl).
1. (general)
A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
entertaining(
ehn
-
uhr
-
tey
-
nihng
)An adjective is a word that describes a noun (e.g., the big dog).
a. entretenido 
The variety show was quite entertaining.El espectáculo de variedades estuvo muy entretenido.
b. divertido 
The mariachis told endless jokes. They were very entertaining.Los mariachis contaron chistes sin parar. Fueron muy divertidos.
c. ameno 
The band at Laura's birthday party was very entertaining.La banda en la fiesta de cumpleaños de Laura fue muy amena.