If we attach another simple sentence, the result is a compound sentence.
1. The polar bear | lives in the Arctic regions, || but || it | sometimes reaches temperate latitudes.
This is manifestly a compound sentence, for it consists of two coordinate clauses, joined by the conjunction but.
The framework of the second clause consists of the subject IT and the simple predicate reaches. To make the complete predicate, the verb reaches takes not only a modifier (the adverb sometimes), but a complement, the direct object latitudes, which completes the meaning of the verb. This noun is itself modified by the adjective temperate. Both clauses are simple, for each contains but one subject and one predicate.
Obviously, almost any number of simple sentences may be joined (with or without conjunctions) to make one compound sentence.
The quiet August noon has come;
A slumberous silence fills the sky;
The fields are still, the woods are dumb,
In glassy sleep the waters lie. - Bryant.
States fall, arts fade, but Nature does not die. - Byron.
The court was sitting; the case was heard; the judge had finished; and only the verdict was yet in arrear. - De Quincey.
He softly blushed; he sighed; he hoped; he feared; he doubted; he sometimes yielded to the delightful idea. - Thackeray.
A mob appeared before the window, a smart rap was heard at the door, the boys hallooed, and the maid announced Mr. Grenville. - Cowper.
His health had suffered from confinement; his high spirit had been cruelly wounded; and soon after his liberation he died of a broken heart. - Macaulay.