A simple sentence has only one independent clause, such as "George ate grapes." An independent clause always contains a subject, which is a noun or pronoun, and a verb.
An independent clause is one that can stand alone. For instance, "George ate grapes," has the subject "George," the verb "ate" and the direct object "grapes," which is what George ate; it is a complete thought. However, if because is added to the sentence, it becomes a dependent clause: "Because George ate grapes." "Because" makes it dependent on another clause to make sense. A simple sentence cannot have a second independent clause or a dependent clause attached to it.