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  • Writer's pictureHannah

Identifying and Fixing Run-On Sentences

Updated: Sep 2, 2021

In my decade of editing experience, I’ve come across countless spelling and grammar errors. From kindergarteners learning about beaver “damns” to Jesus’s “yolk” being easy, no one is immune from the occasional slip-up.

But one type of error comes up quite consistently across skill levels and genres: run-on sentences.

What is a run-on sentence? How do you fix it? Can you ever get away with it? And where does a style guide such as CMOS stand on making exceptions to the rule?

Run-On Sentences Defined

To put it simply, a run-on sentence is when two or more complete thoughts are found in the same sentence but aren’t joined correctly.

Take the following example:

Bernie was cold he wore mittens.

It doesn’t make sense, does it? Two complete thoughts are jammed together. What if you tried using a comma to separate them?

Bernie was cold, he wore mittens.

It might seem better, but there’s still a problem. Commas are usually too weak to separate two complete thoughts on their own; rather, they are best used to connect items in a list or an incomplete thought with a complete one. Editors will usually flag this as an error, especially in formal writing.

If we look at our Bernie example above, the comma is trying to separate two clauses with subjects. This error is called a comma splice, which is a specific type of run-on sentence.

How to Find Run-On Sentences

Are you still unsure if you have a run-on sentence? Try this quick trick.

Split your line into two sentences. If the two thoughts are split by a comma, just make that comma the end of the first sentence. Let’s look at two lines and see how they look when separated into distinct sentences.

Bernie was cold, he wore mittens.

Bernie was cold. He wore mittens.

Since Bernie was cold, he wore mittens.

Since Bernie was cold. He wore mittens.

You can tell the first example was a run-on sentence (a comma splice, to be exact) because the thoughts could both stand independently. The second sentence was not a comma splice, as “Since Bernie was cold” can’t stand alone.

In short, if you can split your sentence in two and it still makes sense, you probably have a run-on sentence. But if one of those sentences sounds awkward, you’re probably connecting a dependent clause to an independent clause correctly.

Now that you can identify run-on sentences, let’s look at how to fix them.


The strongest break you can put between two thoughts is a period. It makes each thought distinct from the other, even if they’re written next to each other.

Bernie was cold. He wore mittens.

In this case, dividing the thoughts into two sentences might not show the relationship between them as clearly as we’d like. For other run-on sentences, using a period might make more sense and help the text flow more smoothly.

We built a snowman yesterday, we rode our bikes around the halls today. (comma splice)

We built a snowman yesterday. We rode our bikes around the halls today. (two sentences)


You might think of commas, semicolons, and periods as pauses in speech. A comma is the briefest of pauses, while a period is the longest pause. The semicolon is somewhere in between; it shows that two things are separate but are closely related ideas.

To use a semicolon to fix a run-on sentence, just put it in between the thoughts.

Bernie was cold; he wore mittens.

Here the relationship between Bernie’s mittens and being cold is a little clearer than when we separated it into two sentences.

Em Dashes

An em dash can be used to show an abrupt change of thought. It brings a bigger emphasis to the second clause than a semicolon.

Bernie was cold—he wore mittens.

Note that this works because the second part is clarifying or expounding upon the first part. This doesn’t work so well with our snowman example, because the two thoughts are much more distinct.

We built a snowman yesterday—we rode our bikes around the halls today.

What does building a snowman have to do with riding bikes? If it’s not readily clear why the two thoughts are related, you might lose your reader.


Sometimes the second thought in your run-on sentence isn’t that important or you want to diminish it rather than draw attention to it. In this case, parentheses might be your best bet.

Bernie wore mittens (he was cold).

Parenthetical statements come across as clarifying notes. You might imagine an actor turning to the audience to give this explanation. It won’t work for every run-on sentence, just like the semicolon doesn’t always fit the situation.

We built a snowman yesterday (we rode our bikes around the halls today).

This probably wouldn't quite work unless someone asked if you rode your bikes yesterday and you’re clarifying which day you did ride them.

Coordinating Conjunctions

If you remember Schoolhouse Rock, you may remember the song, “Conjunction Junction.” Or, if you’re like me, you may just remember the first line and none of the rest of the song. As a refresher, a conjunction is a word that connects thoughts. A coordinating conjunction connects equal words, phrases, or clauses.

If you’re fixing a comma splice, just insert the coordinating conjunction after the comma.

Bernie was cold, so he wore mittens.

Notice how the coordinating conjunction clarifies the relationship between Bernie’s lack of warmth and his mitten usage. Your word choice here can carry a lot of weight, so be sure you’re picking the conjunction that best suits your meaning.


Sometimes run-on sentences just sound better when they’re reworded. This is usually the case with complicated sentences. If you’ve tried the fixes listed above and nothing sounds right, it’s a good sign you should probably just find a new way to state what you’re communicating.

After building a snowman yesterday, we rode our bikes around the halls today.

Notice that our snowman clause can’t stand alone anymore—it has no subject and is, therefore, a dependent clause. While it wasn’t necessary to change the line like that, it added some complexity to the verbiage, thereby making it more interesting while clarifying the relationship between the two clauses. Rewording can be a useful tool, then, for spicing up your writing and engaging your reader.

Are Run-On Sentences Ever Okay?

In creative work, particularly fiction, writers may use grammar that is technically incorrect to produce a specific effect. You’ll note that the Chicago Manual of Style does give exceptions to the rule. Look at the following sentence:

We were pacing, we were ranting, we were tearing out our hair.

This sentence is technically a run-on sentence. But how does it make you feel? The string of thoughts feels rushed, maybe a bit frenzied—which is exactly how the subject feels.

At the hands of an inexperienced writer, this probably won’t work. It’s important to know why you’re breaking rules before you break them. For instance, the poet e. e. cummings routinely did not use capitalization according to grammar rules, including in his own name. Because he was an accomplished writer, his audience understood he was making a point and not simply using poor grammar. If he had been a recent high school graduate with no background in writing, the effect wouldn’t have been the same.

The same is true of intentional run-on sentences. Don’t be offended if your editor marks them as errors. You may just need more experience to prove you know what you’re doing—or you may need to find a better place to break this rule.

Regardless, do note that formal writing requires stricter adherence to standards, so it’s best to avoid run-on sentences in most nonfiction (academics, business, self-help, etc.).

When in Doubt, Fix It

With most run-on sentences, it’s best to default to one of the fixes above. Ridding your formal writing from this nuisance will clarify your meaning, help your words flow more smoothly, and bring your text to a higher level of excellence.

Just keep this blog post handy, and soon your readers will know exactly how mittens relate to Bernie’s temperature.

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