Every sentence that is not simple must be either compound or complex. A sentence is compound if it consists of two or more independent clauses. A sentence is complex, if it consists of one independent (main) clause and one or more subordinate clauses.
An ordinary compound sentence consists of two or more coordinate simple clauses.
Such a sentence may be of great length (as in the last example below). But its structure is usually transparent.
A cricket chirps on the hearth and we are reminded of Christmas gambols long ago. HAZLITT
The moments were numbered. The strife was finished. The vision was closed. DE QUINCEY
The old king had retired to his couch that night in one of the strongest towers of the Alhambra. But his restless anxiety kept him from repose. IRVING.
The clock has just struck two. The expiring taper rises and sinks in the socket. The watchman forgets his hour in slumber. The laborious and the happy are at rest. And nothing wakes but meditation, guilt, revelry and despair. GOLDSMITH
The present, indeed, is not a contest for distant or contingent objects. It is not a contest for acquisition of territory. It is not a contest for power and glory as little is it carried on merely for any commercial advantage or any particular form of government. But it is a contest for the security, the tranquility and the very existence of Great Britain, connected with that of every established government and every country in Europe. PITT
A complex sentence, in its most elementary form, consists of one simple independent (main) clause and one simple subordinate clause.
The gas exploded when I struck a match.
Though he is idle, he is not lazy.
As he is the eldest son, he has to take care of his brothers and sisters.
The carpenter who fell from the roof has recovered from his injuries.
Their eyes were so fatigued with the eternal dazzle and whiteness that they lay down on their backs upon deck to relieve their sight on the blue sky. KEATS
The shouts of thousands, their menacing gestures, the fierce clashing of their arms, astonished and subdued the courage of Vetranio, who stood, amidst the defection of his followers, in anxious and silent suspense GIBBON
Both compound sentences and complex sentences admit of much variety in structure, according to the nature and the relations of the clauses that compose them.