A simple sentence is made up of one independent clause. In order for a clause to be independent, it must be able to stand on its own as a sentence, which means it needs a subject (a noun) and a predicate (a verb).
If we meet these basic requirements, we get short simple sentences:
Simple sentences can also be long and contain phrases that expand the sentence. Remember that phrases lack the subject-verb combination that clauses have. Each example has one subject and one predicate.