The sentence is perhaps the most abused tool in the writer’s arsenal, and the run-on sentence is the most misunderstood of all–the bane of the student writer, the rusty businessman, the ambitious but isolated storyteller. For sentences, a little understanding goes a long way.
The first thing to understand, when it comes to run-ons, is that length is not the problem—the structure is. Plenty of professors fail to grasp this tenet, as they convey to students that shortening sentences will solve the problem and then urge their students toward ever shorter sentences—shorter, even, than Hemingway wrote at his most terse. But the way Hemingway’s some-time laconic prose has grown into legend, exaggerating—even caricaturing—the way he actually did write, the way his sentences actually do wind and turn and expand onto great vistas—well, that’s another post.
Aside from sending you down the other path, the one where you write fragments–so short they aren’t even sentences anymore–nothing is wrong with short sentences, but many things can go right with very long sentences, too. And if you have been deprived of good models by the short-minded among us, take heart. Here is a much-cited sentence by Virginia Woolf, opening her essay “On Being Ill” that is no run-on but that, attempted by a lesser writer, could easily have become one:
“Considering how common illness is, how tremendous the spiritual change that it brings, how astonishing, when the lights of health go down, the undiscovered countries that are then disclosed, what wastes and deserts of the soul a slight attack of influenza brings to view, what precipices and lawns sprinkled with bright flowers a little rise of temperature reveals, what ancient and obdurate oaks are uprooted in us by the act of sickness, how we go down into the pit of death and feel the waters of annihilation close above our heads and wake thinking to find ourselves in the presence of angels and the harpers when we have a tooth out and come to the surface in the dentist’s arm-chair and confuse his ‘Rinse the mouth—rinse the mouth’ with the greeting of the Deity stooping from the floor of Heaven to welcome us—when we think of this, as we are so frequently forced to think of it, it becomes strange indeed that illness has not taken its place with love and battle and jealousy among the prime themes of literature.”
In short, you should not be frightened by length, despite much fearmongering among writing instructors, for sometimes your thoughts are too complex, too sinuous to be squished into a dozen words. Have courage and write your thoughts as they are. Upon your first attempt, I will somewhere be aware of this and raise my glass to you. The world will be better for the more nuanced, inclusive thinking that long sentences encourage.
On a practical level, though, here is what actually goes wrong with your sentences to make them run-ons: The most common type of run-on is a comma splice; you have sewn two perfectly complete clauses (subject + verb) together with a comma. A comma is like a screen or a gauzy curtain thrown down between two things that really deserve their own space. How would you feel if the only thing between you and your neighbor was a bit of sheer fabric? Independent clauses feel the same way.
The best things you can do for yourself (besides taking a hot bath and putting it all into perspective—these really are just sentences, after all, and maybe your English professor does need to get a life), if run-ons are a problem or if you didn’t know they existed, are first, to learn to identify independent clauses, and second, to use something a little more solid than a comma. A period or a semi-colon can serve as the quick-and-easy fix until you develop more mastery over the comma-conjunction one-two punch.
Here is an example of a run-on: My dog chased the cat, I yelled at him to stop.
Brilliant writing, but no one’s going to respect you for it because of what you’ve done to those poor independent clauses. As mentioned above, you have three easy options for the fix:
1) You can just replace the comma with a period. This is a good place to start if you have no confidence and want to build up to more impressive feats of punctuation. It’s also an excellent option if the connection is clear and the two ideas really do feel separate. Example: My dog chased the cat. I yelled at him to stop.
2) Replace the comma with something slightly stronger, such as a semi-colon (;). A semi-colon is useful if the two ideas feel too close to separate them into two different sentences, and if you like the rhythm of the comma. In other words, you get the best of both worlds—the smoothness of the comma with the solidness of a period. Example: My dog chased the cat; I yelled at him to stop. Note: A semi-colon can also be paired with a conjunctive adverb if you want to clarify the connection (some conjunctive adverbs: however, also, thus, in addition, therefore, consequently). Think of it as the high-brow version of the comma + conjunction team (below).
3) Finally, keep the comma and add a coordinating conjunction (and, but, for, so, or, nor, yet). This is a good choice if you want to make the connection clear to the reader because each of these conjunctions expresses a specific type of connection. Example: My dog chased the cat, and I yelled at him to stop. (“So” could also work, if you wished to show cause-effect.)
 In all fairness to your beleaguered writing instructors, past and present, it takes considerable knowledge of phrases and subordinate clauses to build a sentence of such great length without run-on problems. Still, you deserve to be taught all those phrases and clauses, so you have the option.