A run-on sentence is not simply a long sentence. Long sentences may be well balanced and perfectly grammatical, as the sentence from Dickens' Hard Times discussed on the Myths page illustrates. At a minimum, a sentence in English contains one clause - that is, a group of words possessing a subject and a predicate. When a clause can stand by itself as a complete sentence, it is said to be independent. Independent clauses must be joined by either a conjunction or appropriate punctuation. (Conjunctions are words such as and, but, yet, although).
Most run-ons occur when a writer joins independent clauses with no conjunction or punctuation, or with a comma (see comma splice). In general, independent clauses joined by punctuation require either a colon or a semicolon. They can also, of course, be written as two sentences, each beginning with a capital letter and ending with a period. The following is an example of a run-on sentence. It contains two independent clauses. It also contains two subordinate (aka "dependent") clauses. (A subordinate clause has a subject and a predicate but could not stand alone as a sentence.) The subordinate clauses appear below in boldface.
The writer has used a comma (following partner) to join the first independent-subordinate combination to the second. In order to correct the error, the writer could (1) replace the comma with a semicolon or conjunction (and would do) or (2) begin a new sentence after partner.
Neither solution, however, would leave the reader with a very clear sense of logical relationships. Editing run-ons offers the writer an opportunity to eliminate wordiness and enhance directness. Compare the original to the following revision: