A complete sentence needs a subject and a verb and it has to express a complete thought. "Birds (noun) fly (verb)"'is a simple sentence. We have compound sentences: "Birds fly and fish swim." We have complex sentences: "Birds fly south when the summer ends." We have compound - complex sentences: "Birds fly south when the summer ends but only if they have a complete set of feathers." An independent can stand by itself as a simple sentence. An independent clause has a subject and predicate and can stand alone as a main clause. You can have two independent clauses in one sentence; however, you require a coordinating conjunction such as "but" or proper punctuation such as a coma or semi colon. "Birds like flying but fish like swimming" (birds like flying/fish like swimming). "I am going to a vacation resort; I intend to stay at the resort for two weeks."

Both have a subject and main verb. However, a sentence can be composed of more than one clause, the other clauses being independent and/or dependent.