It’s been my experience that when people see a very long sentence they immediately decide it’s a run-0n. In fact, you can have one sentence comprising thousands of words (even though no one would possibly want this), and it would not be a run-on, as long as it was structured correctly.
A run-on is a complete sentence, no matter how long or short, that is joined to another complete sentence by two different means:
- Jim is tall his brother is shorter. Here you have two complete sentences that have nothing to join them. This is the classic run-on.
- Jim is tall, his brother is shorter. Here the two sentences are joined by a comma, making what is known as a comma splice, another form of a run-on.
It’s easy to fix run-ons.
- You can put a period between the two sentences: Jim is tall. His brother is shorter. With very short sentences like these, using a period may seem a bit simplistic, but it’s not wrong.
- You can also use a semicolon between the two sentences, assuming they are closely related in subject matter: Jim is tall; his brother is shorter.
- You can add a connecting word: Jim is tall although his brother is shorter.
We most often write run-ons when we’re in a hurry. If we don’t take time to proofread (audibly—quietly so you can hear your own voice—and slowly), chances are we won’t catch them. But our readers may, and it’s best not to let that happen. It may not be fair, but we are often judged by our writing.