Here's why:
* Noun: A word that names a person, place, thing, or idea. "Trees" refers to a type of plant.
* Pronoun: A word that replaces a noun (e.g., he, she, it, they). "Trees" doesn't replace a noun.
* Adjective: A word that describes a noun (e.g., big, red, happy). "Trees" doesn't describe anything.
Let me know if you have more grammar questions!