Here's why:
* Nouns are words that name people, places, things, or ideas (e.g., dog, park, book, happiness).
* Pronouns are words that replace nouns (e.g., he, she, it, they).
* Determiners are words that come before nouns to specify which noun is being referred to (e.g., a, an, the, this, that, my, your).
In the sentence "A dog is running," the word "a" specifies which dog is being talked about.