"In the backyard, the dog barked and howled at the cat" is an example of—

This is a simple sentence because it has one independent clause with one subject (the dog) and a compound verb (barked and howled). Although it has two verbs joined by 'and', they share the same subject, making it a simple sentence with a compound predicate. A complex sentence would have a dependent clause, and a compound sentence would have two independent clauses. The prepositional phrase 'in the backyard' is just a modifier.