Here's why:
* Pronouns stand in for nouns (e.g., he, she, it, they).
* Nouns are words that name people, places, things, or ideas (e.g., dog, park, book, happiness).
* Verbs express actions or states of being (e.g., run, jump, think, exist).
"A" is a determiner that specifies a singular, non-specific noun. For example:
* A dog barked. ("Dog" is the noun)
* I saw a car. ("Car" is the noun)