Here's why:
* Compound sentence: It has two or more independent clauses joined by a coordinating conjunction (like "and", "but", "or").
* Compound subject: The subject of the sentence is made up of two or more nouns or pronouns joined by a conjunction.
Example:
* The cat and the dog played in the garden.
Here, "The cat and the dog" are the compound subject, and "played" is the verb that applies to both.