Two very different things get called run-on sentences: fused sentences and comma splices. Careful academic and business readers just hate them. Whether it's fair or not, readers often see these non-standard structures as evidence that the writer is ignorant and probably not worth hiring. Not only that, but these non-standard sentences are sometimes very unclear and always have a flat, immature style.
The discussion of run-on sentences is all about sentence structure, not length or meaning, so don't retreat into "Well, I could figure it out." That's not the point.
Some of my students have been given fake "rules" about maximum length (usually quite short), but even a very short sentence can be a run-on sentence:
Jon loves to run some other runners don't.
Good business writing has sentences in the 20+ word range and heavy academic writing uses sentences that are even longer, so don't just count words. That's not how you find run-ons (no matter what an ill-informed teacher ever told you).
A sentence can be structurally excellent and still be either meaningless or total nonsense. It's all about structure, remember. Here's a meaningless sentence (that requires a context to define its pronouns) and a nonsense sentence (that uses words that are undefined):
Each wants one, and both want the other.
It was brillig and the slithy toves did gyre and gimbal on the wabe.
Both sentences are structurally excellent and impossible to understand.
We are in love we plan to get married.
Two structurally complete sentences (that is, two independent clauses) are presented as if they are one without any punctuation or other way of joining them. Most student writers would recognize that something has gone wrong here if it's pointed out to them.
Waldo is a wonderful student, he graduated with honors.
This is extremely common in student writing, and it is usually intentional. If I point out that the student has tried to join two sentences with a comma (which simply doesn't work), the student usually cannot figure out what the problem is. Yes, you see comma splices in a lot of informal writing (J.K. Rowling does a lot of them), but that doesn't mean they are acceptable in formal writing.
Of course, you can always supply end punctuation and capital letters ("We are in love we plan to get married." = "We are in love. We plan to get married."), but that will generate a lot of short, choppy sentences, and you probably put everything in one structure because you were convinced that the material belonged together. Here are four ways to get two independent clauses into one sentence correctly.
A few grammar checkers can sometimes spot run-on sentences, but they are very poor at it—they often simply count words. You've got to learn how to spot independent clauses yourself.
I will usually mark R.O. above the place where the problem occurred. I do not mark C.S. above comma splices because students take out the comma, and that just makes things worse.
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Revised 8/10/17 • Page author: Curtis Allen • e-mail: email@example.com.