The boy eats an apple. "el niño se come una manzana".
She is an honest woman. "Es una mujer honesta".
Topic principal
What are then articles?
According to the Oxford Dictionaries it is a determiner that introduces a noun phrase. a | an are considered indefiniten articles and they are used to introduce new concepts intro a discourse. Or a scheme.
Use the following words. 1- use "a" only with singular. 2- use "an" whith words that with a vowel sound.