The subject of a simple sentence may consist of one noun or pronoun with its modifiers, or of two or more nouns with their modifiers. In the latter case, the subject is called a compound subject. Similarly, the predicate may contain one verb or two or more verbs. Occasionally a sentence will have a compound subject and a compound predicate, as in the last example given.
Compound Sentences
A compound sentence is one which consists of two or more assertions of equal grammatical value—that is, neither assertion is grammatically subordinate to the other or to any part of the other.
The rain descended, and the floods came.
The automobile is fast, but the airplane is faster.
Complex Sentences
A complex sentence is one which contains not only a main assertion (or more than one), but also one or more subordinate assertions—that is, it contains one or more assertions which are grammatically incorporated as a noun or an adjective or an adverb into the main clause or into another subordinate clause.
I saw that he had finished.
Heavy automobiles which carry large loads run smoothly.