Here's why:
* Noun: A noun is a word that names a person, place, thing, or idea. It's the core of the subject, telling us *who* or *what* is performing the action.
* Example: "The cat chased the mouse." (Cat is the noun and the main word in the subject)
* Pronoun: A pronoun is a word that replaces a noun. It can also be the main word in a subject.
* Example: "He chased the mouse." (He is the pronoun and the main word in the subject)
Let me know if you'd like more examples!