Run-Ons, Comma Splices, and Fragments
These three errors are some of the most common mistakes in college writing—but once you see how they work, they’re easy to spot and fix.
Here’s the big picture. Every sentence needs three things: a subject (who or what the sentence is about), a main action (what’s happening), and an ending (a period, question mark, or exclamation point). [Editor note: Technically, a semicolon can also end a sentence, but even professional writers disagree on how to use semicolons correctly—so we’ll skip that and keep things simple.] When those three pieces get out of balance, you end up with one of these problems:
- A run-on happens when you jam two complete sentences together without any punctuation or conjunction between them. It’s two sentences pretending to be one.
- A comma splice is almost the same thing, except you stick a comma between the two sentences. The problem is that a comma isn’t strong enough to hold two complete sentences together on its own. It’s still two sentences pretending to be one.
- A fragment is the opposite problem—it’s a collection of words that’s missing something important and can’t stand on its own. It’s a group of words pretending to be a sentence.
The good news is that there are simple ways to find and fix all three of these errors. One is to read your writing out loud—your ear will often catch what your eye misses, because you’ll naturally pause or stumble where something isn’t right. Another is to look more closely at structure and meaning—counting up the subjects, main actions, and endings in your sentences to make sure everything balances out. We’ll walk through both strategies below.
Run-On Sentences
A run-on sentence is what happens when two complete sentences are stuck together with nothing between them. It’s two sentences pretending to be one.
Consider this example:
“I like basketball Amy likes swimming.”
There are really two separate sentences here: “I like basketball” and “Amy likes swimming.” But there’s no period, no conjunction—nothing to show where one stops and the other starts. That’s a run-on.
Here’s another one:
“She loves to read she buys a new book every week.”
Again, two complete sentences crammed together with nothing separating them. The reader has to figure out on their own where the break should go, and that’s not their job—it’s the writer’s.
Spotting Run-Ons
So how do you catch run-ons in your own writing? Two strategies.
First, read it out loud. Read that first example—”I like basketball Amy likes swimming”—and listen for what happens. Your voice will naturally want to stop after “basketball,” because that’s where one thought ends and a new one begins. When you feel your voice hit a wall like that, there’s a good chance you need a period or a conjunction.
Second, try the Balance Check. A properly built sentence has one subject, one main action, and one period or conjunction. When those numbers don’t match, something is off. Let’s count:
“I like basketball Amy likes swimming.”
- Subjects: two (I, Amy)
- Main actions: two (liking basketball, liking swimming)
- Periods or conjunctions: zero
Two subjects and two main actions, but nothing to properly separate or connect them. The numbers don’t match—that’s your signal something is wrong.
What Counts as a “Main Action”?
Ask yourself: How many separate things are happening in this sentence?
Take the sentence “She sat down and cried.” At first glance, it might look like two things are happening—sitting down and crying. But step back and think about what’s really going on. This is one moment: someone is upset. She sat down and cried—those two action words are working together to describe one situation, one main thing happening. That’s one main action.
Now compare that to “I like basketball Amy likes swimming.” Here, two completely separate things are happening. One person likes one sport, and a different person likes a different sport. Those aren’t working together to describe one situation—they’re two unrelated main actions. That’s your mismatch.
Fixing Run-Ons
Once you’ve spotted the run-on, fixing it is straightforward. One option is to simply add a period and make them two separate sentences:
Run-on: “She loves to read she buys a new book every week.”
Fixed: “She loves to read. She buys a new book every week.”
The other option is to add a comma and a conjunction. There’s an easy way to remember which conjunctions can do this job: FANBOYS. That stands for For, And, Nor, But, Or, Yet, So. When you pair a comma with one of these words, the comma and the conjunction work as a team to join two complete sentences together.
Run-on: “I like basketball Amy likes swimming.”
Fixed: “I like basketball, and Amy likes swimming.”
Remember: the comma and the FANBOYS word need each other. A conjunction alone without a comma before it doesn’t do the job either. They work as a team.
Comma Splices
A comma splice looks almost right—but it’s not. It happens when you put a comma between two complete sentences without a conjunction. Like a run-on, it’s two sentences pretending to be one.
Why Commas Can’t Do This Job
To understand why this doesn’t work, it helps to think about what commas actually do. Commas are used for things like separating items in a list (“apples, bananas, and oranges”), setting off introductory words (“Yesterday, I went to the store”), and adding extra information into the middle of a sentence (“My dog, who is very old, still loves to play”). What all of these have in common is that the comma is working inside a single sentence. A comma is not built to stand between two complete sentences—that’s a job for a period, or for a comma plus a conjunction.
Look at this example:
“I like basketball, Amy likes swimming.”
This is so close to being correct. The writer clearly felt that a pause belonged there, and they were right! But a comma by itself isn’t strong enough to hold two complete sentences together. It’s like trying to hold a door shut with tape—it might look like it’s working, but it won’t hold up.
Here’s another comma splice:
“I’m not mad, I’m just disappointed.”
Two complete sentences. Two separate things being said. A comma alone can’t do the job here.
Spotting Comma Splices
You can catch comma splices the same two ways you catch run-ons. Read it out loud, and listen for whether your voice drops to a full stop where the comma is. If it does, the comma probably isn’t enough. Then try the Balance Check:
“I like basketball, Amy likes swimming.”
- Subjects: two (I, Amy)
- Main actions: two (liking basketball, liking swimming)
- Periods or conjunctions: zero—a comma doesn’t count
Two subjects and two main actions, but no period or conjunction. The comma is there, but it can’t do this job by itself. Mismatch—that’s a comma splice.
The Comma Rule
A comma alone cannot separate two complete sentences. You need a period, or a comma plus a conjunction. If you remember nothing else from this lesson, remember this.
Fixing Comma Splices
Fixing a comma splice works the same way as fixing a run-on. You can add a period:
Comma splice: “I’m not mad, I’m just disappointed.”
Fixed: “I’m not mad. I‘m just disappointed.”
Or you can add a FANBOYS conjunction after the comma that’s already there:
Comma splice: “She loves to read, she buys a new book every week.”
Fixed: “She loves to read, so she buys a new book every week.”
Fragments
A fragment is a group of words pretending to be a sentence. It’s missing a subject, a main action, or both.
Every sentence needs a subject (who or what you’re talking about) and a main action (what’s happening). If either one is missing, the words don’t form a complete sentence—they form a fragment.
Here’s a fragment that’s missing a subject:
“Loves omelettes with ham, cheese, and mushroom.”
There’s an action here (loves), but who loves omelettes? We have no idea. There’s no subject.
Here’s a fragment that’s missing a main action:
“A blue house next to a red house on the corner of Elm Street.”
That’s a very detailed description, but nothing is happening. What about these houses? Were they built? Sold? Demolished? There’s no action.
The Tricky Ones
And here’s a tricky kind of fragment—one that has a subject and an action but still isn’t a complete sentence:
“After I get home.”
After you get home… what? You’ll take a nap? Watch TV? Eat dinner? This group of words leaves the reader hanging. Compare it to “I get home,” which is a complete sentence—the word “after” changes everything by turning it into a setup that needs a payoff.
Here’s another one like it:
“Because she loves to read.”
That “because” at the beginning sets up an expectation: because she loves to read… she goes to the library? She wants a Kindle for her birthday? The words never deliver on their promise.
Wait—Are Commands Fragments?
You might look at a sentence like “Sit down” and think it’s a fragment because there’s no subject. But commands are not fragments. They’re complete sentences. The subject is “you”—it’s just hidden. The person reading or hearing the command understands exactly who is being told to do something, even though the word “you” isn’t written out.
Here’s what’s really going on:
“Sit down.” → really means “You sit down.”
“Close the door.” → really means “You close the door.”
“Turn in your essays by Friday.” → really means “You turn in your essays by Friday.”
Each of these has a subject (you), a main action (sit, close, turn in), and an ending (the period). The subject is just understood rather than written out. So don’t mistake short commands for fragments—they have everything a sentence needs.
Spotting Fragments
Catching fragments is a little different from catching run-ons and comma splices. Reading out loud still helps—fragments often sound like you’re leaving a sentence hanging, and you’ll hear that unfinished feeling in your voice. But the Balance Check works differently here. Instead of finding a mismatch where the numbers are too high, you’re looking for what’s missing: a subject with no action, an action with no subject, or words that feel like a setup with no payoff.
Fixing Fragments
Fixing a fragment usually means adding whatever is missing or attaching the fragment to a nearby sentence:
Fragment (missing a subject): “Loves omelettes with ham, cheese, and mushroom.”
Fixed: “Marcus loves omelettes with ham, cheese, and mushroom.”
Fragment (missing a main action): “A blue house next to a red house on the corner of Elm Street.”
Fixed: “A blue house next to a red house on the corner of Elm Street caught her attention.”
Fragment (incomplete setup): “Because she loves to read.”
Fixed: “Because she loves to read, she goes to the library every Saturday.”
The Owner’s Manual Test
Imagine your sentences are steps in an owner’s manual. Each step should make complete sense on its own. For example:
Step 1: Remove the filter from the back of the machine.
Step 2: Rinse the filter under warm water.
Step 3: With a dry cloth before replacing.
Steps 1 and 2 make perfect sense—you know exactly what to do. But Step 3 leaves you confused. With a dry cloth before replacing… what? Wipe it? Dry it? There’s no action, and the reader is left guessing. That’s a fragment. If a group of words wouldn’t work as a step in a manual, it probably can’t stand on its own as a sentence either.
Quick Summary
Run-on: Two complete sentences stuck together with nothing between them. Fix it by adding a period or a comma with a FANBOYS conjunction.
Comma splice: Two complete sentences joined by only a comma. A comma can’t do that job alone. Fix it the same way you’d fix a run-on.
Fragment: A group of words pretending to be a sentence. It’s missing a subject, a main action, or both. Fix it by adding what’s missing or by attaching it to a nearby sentence.
The Balance Check: Count your subjects, main actions, and periods or conjunctions. If the numbers don’t match, take a closer look.
Click on the video above for more examples and explanations of how to identify and fix these common sentence errors.