Run-Ons and Comma Splices — Correct a run-on sentence or...

SECTION 1

Why Sentence Boundaries Matter

Have you ever tried reading a text message that had no periods or breaks? It can be really confusing. For thousands of years, writers have struggled with the same problem: how do you show where one idea ends and the next begins? The rules we follow today took a long time to develop.

Understanding this history helps you see why run-on sentences and comma splices are considered errors. They make writing harder to read. On the SHSAT, you will be asked to find and fix these mistakes in passages.

~200 BCE

Ancient Greek Punctuation

Greek librarians in Alexandria began using dots at different heights to mark pauses in texts. Before this, words ran together with no spaces or stops at all.

~700 CE

Spaces Between Words

Irish and Anglo-Saxon monks started adding spaces between words so that readers of Latin could understand texts more easily.

1400s–1500s

The Printing Press Era

After the printing press was invented, punctuation marks like the period, comma, and semicolon became standardized across Europe.

1800s–1900s

Modern Grammar Rules

Grammar textbooks established clear rules: every complete thought (independent clause) needs proper punctuation at its boundary. Run-ons and comma splices became officially recognized errors.

Today, clear sentence boundaries are a basic expectation in formal writing. The SHSAT tests whether you can recognize when two complete thoughts have been improperly joined. So the key question is: how do you tell when sentences are crashing into each other, and what are the correct ways to fix them?

SECTION 2

Core Definitions and Principles

Before you can fix run-ons and comma splices, you need to understand what an independent clause is. An independent clause is a group of words that has a subject (who or what), a verb (what happens), and expresses a complete thought. For example, "The dog barked" is an independent clause. It can stand on its own as a sentence.

Independent Clause

A group of words with a subject and a verb that expresses a complete thought. It can be a sentence by itself. Example: Maria studied all night.

Run-On Sentence

Two or more independent clauses joined together with no punctuation or connecting word at all. Example: Maria studied all night she felt prepared.

Comma Splice

Two independent clauses joined with only a comma but no connecting word. The comma is not strong enough by itself. Example: Maria studied all night, she felt prepared.

Four Ways to Fix Them

You can fix a run-on or comma splice by using (1) a period, (2) a semicolon, (3) a comma + coordinating conjunction, or (4) a subordinating conjunction to make one clause dependent.

✦ KEY TAKEAWAY

Think of independent clauses like two cars on the road. A run-on is like two cars smashing into each other with no traffic signal. A comma splice is like a flashing yellow light — it slows traffic but doesn't properly direct it. You need a real traffic signal (a period, semicolon, or conjunction) to keep everything flowing safely.

SECTION 3

Seeing the Problem — A Visual Guide

The diagram below shows you how to identify a run-on and a comma splice, and the four correct ways to fix them. Follow the arrows from the error at the top to the solutions at the bottom.

This flowchart shows the two types of errors (run-on and comma splice) at the top, and the four correct methods for fixing them below. Notice how each fix uses a different tool — a period, semicolon, comma with conjunction, or subordinating word — to properly separate or connect the two independent clauses.

Look at the diagram carefully. The two boxes at the top show the two types of errors. A run-on (fused sentence) has absolutely nothing between two complete thoughts. A comma splice has a comma, but a comma alone is not strong enough to hold two independent clauses together. The four green-bordered boxes below show the correct fixes you can use.

SECTION 4

How Each Fix Works

Fix 1: Use a Period

The simplest fix is to split the sentence into two separate sentences with a period. This works every time. Just put a period after the first complete thought and start a new sentence with a capital letter. For example: The library was crowded. We found a quiet corner.

Fix 2: Use a Semicolon

A semicolon (;) is like a soft period. It tells the reader, "These two ideas are closely related, so I want to keep them together." Use it when the two clauses are about the same topic. For example: The library was crowded; we found a quiet corner. Note that you do not capitalize the word after a semicolon (unless it is a proper noun).

Fix 3: Use a Comma + Coordinating Conjunction (FANBOYS)

You can join two independent clauses with a comma followed by a coordinating conjunction. The seven coordinating conjunctions spell out FANBOYS: For, And, Nor, But, Or, Yet, So. The comma goes right before the conjunction. For example: The library was crowded, but we found a quiet corner. Remember: the comma alone is not enough. You need both the comma AND the FANBOYS word.

Fix 4: Use a Subordinating Conjunction

A subordinating conjunction (a word like "because," "although," "when," "since," or "if") turns one independent clause into a dependent clause (a clause that cannot stand on its own). Now you no longer have two independent clauses crashing together. For example: Although the library was crowded, we found a quiet corner. When the dependent clause comes first, put a comma after it. When it comes second, you usually do not need a comma.

💡 SHSAT TIP

On the SHSAT, the answer choices will often offer all four fixes. Your job is to pick the one that makes the sentence read most smoothly and clearly. Look for the choice that keeps the original meaning while using correct punctuation.

SECTION 5

Tricky Cases and Common Traps

Some sentences look correct but are actually comma splices in disguise. Others look like run-ons but are actually fine. Let's explore the cases that trip students up most often on the SHSAT.

This reference chart shows the four most common traps students encounter on the SHSAT. Trap 1 involves confusing conjunctive adverbs (like however) with coordinating conjunctions. Trap 2 reminds you that a long sentence is not automatically a run-on. Trap 3 warns that even short sentences can be run-ons. Trap 4 confirms that a comma after an introductory dependent clause is perfectly correct.

The golden rule to remember is simple: test each part on its own. Cover up everything after the comma (or the spot where you think the break is) and ask yourself, "Could this part be its own sentence?" Then check the second part. If both parts are complete sentences, you need more than just a comma to join them.

SECTION 6

Worked Example — SHSAT Style

Let's walk through an SHSAT-style question step by step. Imagine you see this sentence in a passage on the test:

📝 TEST SENTENCE

The ancient Romans built roads across their empire, these roads connected cities and helped trade.

Correcting a Comma Splice Step by Step

Step 1 — Identify the ClausesRead the sentence and look for two complete thoughts. The first part is "The ancient Romans built roads across their empire." It has a subject (Romans), a verb (built), and it expresses a complete thought. The second part is "these roads connected cities and helped trade." It also has a subject (roads), a verb (connected), and is a complete thought.

Two independent clauses identified.

Step 2 — Check What Joins ThemLook at the spot where the two clauses meet. There is only a comma between "empire" and "these." Is there a coordinating conjunction (FANBOYS word) after the comma? No. This means the comma is doing all the work by itself, which is not allowed.

This is a comma splice.

Step 3 — Consider the Fix OptionsWe have four choices. (A) Period: "...empire. These roads connected..." (B) Semicolon: "...empire; these roads connected..." (C) Comma + conjunction: "...empire, and these roads connected..." (D) Subordination: "...empire because these roads connected..." All four are grammatically correct, so we pick the one that sounds best and keeps the original meaning.

Multiple valid fixes exist — choose the smoothest one.

Step 4 — Select the Best AnswerThe second clause adds related information about the roads, so a semicolon or a comma with "and" both work well. On the SHSAT, look at the answer choices given. If the choices include "empire, and these roads," that is a strong pick because it smoothly connects two related ideas.

Best answer: "The ancient Romans built roads across their empire, and these roads connected cities and helped trade."

SECTION 7

Comparing the Four Fixes

Each fix has its own strengths and best uses. The table below will help you understand when to use each one. On the SHSAT, the test writers usually offer only one grammatically correct answer — but knowing the strengths of each fix helps you eliminate wrong choices faster.

Comparison of the four methods for fixing run-on sentences and comma splices.

Fix Method	When to Use It	Watch Out For
Period (.)	When the two ideas are somewhat separate or when you want short, punchy sentences.	Too many short sentences in a row can sound choppy. Vary your style.
Semicolon (;)	When the two ideas are closely related and you want to show that connection without a conjunction.	Do not capitalize the word after a semicolon unless it is a proper noun.
Comma + FANBOYS	When you want to show a specific relationship (contrast with "but," addition with "and," result with "so").	You must use BOTH the comma AND the conjunction. A conjunction alone without a comma before it can also be an error.
Subordination	When one idea depends on or explains the other (cause/effect, time, condition).	When the dependent clause comes first, follow it with a comma. When it comes second, a comma is usually not needed.

✦ KEY TAKEAWAY

Think of the four fixes like four different tools in a toolbox. A period is like scissors — it cuts cleanly. A semicolon is like a hinge — it connects while still showing a break. A comma + FANBOYS is like glue with a label — it joins and tells you the relationship. And subordination is like nesting one box inside another — one idea becomes part of the other.

SECTION 8

Looking Ahead — Sentence Combining in High School

Once you master identifying and fixing run-ons and comma splices, you are ready for more advanced sentence-combining skills. In high school English classes (and on tests like the SAT), you will encounter even more complex situations. Here is a preview of how your skills will grow.

How SHSAT-level skills connect to advanced grammar topics.

What You Learn Now (SHSAT)	What Comes Next (High School & SAT)
Recognize run-ons and comma splices	Fix errors in passages with multiple paragraph contexts
Use the four basic fixes (period, semicolon, comma + FANBOYS, subordination)	Choose the fix that best matches the author's tone, style, and purpose
Know FANBOYS conjunctions	Use colons, dashes, and parenthetical expressions to combine sentences
Identify independent and dependent clauses	Analyze sentence structure including appositives, participial phrases, and absolutes

The skills you are building right now are the foundation. Every advanced grammar concept builds on your ability to recognize independent clauses and understand how they should be connected. Think of it like learning basic addition before you can do algebra — you need this first.

SECTION 9

Practice Problems

Try these five problems. They get harder as you go. For each one, decide if the sentence is correct, a run-on, or a comma splice — and then think about how to fix it.

PROBLEM 1 — CONCEPTUAL

Which of the following is a comma splice? (A) After the storm ended, the sky cleared. (B) The storm ended, the sky cleared. (C) The storm ended, and the sky cleared. (D) The storm ended. The sky cleared.

PROBLEM 2 — BASIC

Fix this run-on sentence: "The subway was delayed we had to take the bus instead." Which correction is best? (A) The subway was delayed, we had to take the bus instead. (B) The subway was delayed we, had to take the bus instead. (C) The subway was delayed, so we had to take the bus instead. (D) The subway, was delayed we had to take the bus instead.

PROBLEM 3 — INTERMEDIATE

Read this sentence: "The museum had a special exhibit on dinosaurs, however, it was only open on weekends." What is the error, and how should it be corrected? (A) No error. (B) Change the comma before "however" to a semicolon. (C) Remove "however" entirely. (D) Change "however" to "and."

PROBLEM 4 — APPLIED

You are editing a classmate's essay and find this paragraph: "New York City has over 800 languages spoken within its borders it is the most linguistically diverse city in the world. Researchers study these languages, they want to understand how communities communicate." Identify all the errors and rewrite the paragraph correctly.

PROBLEM 5 — CRITICAL THINKING

A student claims: "You can always fix a comma splice by simply removing the comma." Is this statement true? Explain your reasoning with an example.

SUMMARY

Lesson Summary

A run-on sentence occurs when two independent clauses are joined with no punctuation at all. A comma splice occurs when two independent clauses are joined with only a comma and no conjunction. Both errors confuse readers by failing to properly mark where one thought ends and the next begins.

You can fix these errors in four ways: use a period to make two separate sentences, use a semicolon for closely related ideas, use a comma plus a FANBOYS conjunction (For, And, Nor, But, Or, Yet, So), or use a subordinating conjunction to turn one clause into a dependent clause. On the SHSAT, always test whether both parts of a sentence can stand alone. If they can, make sure they are properly joined. Watch out for conjunctive adverbs like "however" and "therefore" — they are not conjunctions and cannot join clauses with just a comma.