vs
QUICK ANSWER
"Pronoun" is a noun which is often translated as "el pronombre", and "once" is an adverb which is often translated as "una vez". Learn more about the difference between "pronoun" and "once" below.
pronoun(
pro
-
naun
)A noun is a word referring to a person, animal, place, thing, feeling, or idea (e.g., man, dog, house).
1. (grammar)
a. el pronombre (M)
(m) means that a noun is masculine. Spanish nouns have a gender, which is either feminine (like la mujer or la luna) or masculine (like el hombre or el sol).
I think your story would sound more personal if you used more proper names and fewer pronouns.Creo que tu cuento sonaría más personal si usaras más nombres propios y menos pronombres.
once(
wuhns
)An adverb is a word that describes a verb, an adjective, or other adverbs (e.g., to run quickly, very tired).
2. (formerly)
a. antes 
I once knew how to play the piano, but I've forgotten almost everything.Antes sabía tocar el piano, pero lo he olvidado casi todo.
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Once a liar, always a liar.El que miente una vez, siempre será mentiroso.
Once a teacher, always a teacher.Una vez que se es profesor, se es profesor para siempre.
A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
4. (when)
a. una vez que 
He fell exhausted to the ground once he crossed the finish line.Cayó exhausto al suelo una vez que cruzó la meta.
A noun is a word referring to a person, animal, place, thing, feeling, or idea (e.g., man, dog, house).
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
I decided to agree to it this once.Decidí permitirlo solo por esta vez.
I only saw him the once.Solo lo vi una vez.