Which sentence is an example of a simple sentence?

A simple sentence is characterized by having a single independent clause that contains a subject and a predicate, and it expresses a complete thought. The sentence "Mary went to the store" fits this definition perfectly as it has a clear subject ("Mary") and a verb ("went"), thereby presenting a full idea without any additional clauses or phrases that could complicate it.

In the other examples, the sentences contain more than one clause: the second one features two independent clauses connected by the conjunction "and," making it a compound sentence. The third sentence includes a dependent clause introduced by "because," which means it cannot stand alone as a complete thought, making it a complex sentence. The fourth option has a compound structure due to having two subjects ("Mary and John") but still remains a simple idea; however, the presence of the word "together" does not change its complexity compared to the straightforward nature of the first sentence. Thus, the simplest and clearest example of a simple sentence is the first option.