Here's why:
* Simple subject: This is the main noun or pronoun that performs the action of the verb. It's the core of the sentence, the thing or person doing something.
* Compound subject: If you have multiple nouns or pronouns that are both performing the action of the verb, they become a compound subject. They are joined by a coordinating conjunction (like "and", "or", "but").
Examples:
* Simple subject: The dog barked.
* Compound subject: The dog and the cat barked.
While you can have multiple nouns in a sentence, if they are not working together to perform the verb's action, they are likely part of a prepositional phrase or other grammatical structure, not the subject.