Here's why:
* Nouns are words that name people, places, things, or ideas. Examples: *dog, city, happiness, John*
* Pronouns are words that stand in for nouns. Examples: *he, she, it, they, we*
Let's look at some examples:
* The dog barked loudly. (Subject: *dog*, a noun)
* She went to the store. (Subject: *she*, a pronoun)
* My brother and I are going to the movies. (Subjects: *brother* and *I*, both nouns and a pronoun)
While the subject of a sentence is usually a noun or a pronoun, it's important to note that it can also be a noun phrase or a pronoun phrase:
* The big, brown dog barked loudly. (Subject: *The big, brown dog*, a noun phrase)
* All of them went to the store. (Subject: *All of them*, a pronoun phrase)