Here's why:
* Nouns are words that name people, places, things, or ideas (e.g., dog, park, book, happiness).
* Pronouns are words that stand in for 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, these, those).
In the sentence "A dog is running," "dog" is the noun, and "a" tells us that it's one specific, unidentified dog.