Here's why:
* Nouns are words that name people, places, things, or ideas. For example: *dog, city, book, happiness*.
* Verbs are words that describe actions or states of being. For example: *run, eat, sleep, be*.
* Indefinite articles (like "a" and "an") are used before singular countable nouns to indicate that we're talking about any one of that type of thing. For example: "a dog," "an apple."
Let me know if you'd like more examples!