A compound sentence is a sentence with at least two independent clauses and no dependent clauses. What’s an independent clause? It’s a phrase that can stand alone as a sentence. “I slept late” is an independent clause. So are “The sun is shining” and “Ella got a new dog.” Each has a subject and a verb and makes sense on its own. Examples of dependent clauses, which can’t be complete sentences on their own include: “whenever I go to the lake,” “how she got lost,” and “what makes him happy.” They don’t make sense on their own.
A compound sentence must contain at least two independent clauses. Here’s an example: “I practiced piano every day, yet I never got very good at it.” The two independent clauses are “I practiced piano every day” and “I never got very good at it.” They are joined by a comma and the coordinating conjunction “yet.” Both clauses have a subject and verb and make sense on their own, and since there are no dependent clauses, we know this is a compound sentence.