The Sentence

Lesson 8: Run-On Sentences and Comma Splices

Lesson: 8 of 9 | Level: 🟢 Elementary

1. Lesson Overview

If a sentence fragment is a sentence that stops too soon, a run-on sentence is a sentence that does not stop when it should. It occurs when two or more independent clauses are joined without the punctuation or conjunction needed to make the relationship between them clear and grammatically correct.

The comma splice is a closely related error — perhaps the most common punctuation mistake in written English. It occurs when two independent clauses are joined by a comma alone, without a coordinating conjunction or other appropriate linking device.

Both errors suggest to the reader that the writer has not fully controlled the boundaries of the sentence — where one idea ends and another begins. This lesson identifies both errors precisely, explains why they occur, and provides clear, practical strategies for correcting them.

Objectives

By the end of this lesson, you will be able to:

• Define a run-on sentence and explain why it is a grammatical error
• Define a comma splice and distinguish it from a run-on sentence
• Apply four strategies for correcting run-on sentences and comma splices
• Recognise and avoid both errors in your own writing

2. Core Content

A. Defining a Run-On Sentence

A run-on sentence occurs when two or more independent clauses are written together without any punctuation or conjunction between them. The clauses simply collide — one ends and the next begins with no signal to the reader that a boundary has been crossed.

Consider the following:

The Amazon is the world’s largest river by discharge the Nile is the world’s longest river. Glaciers are retreating worldwide sea levels are rising as a result scientists are concerned.

In the first example, two complete, self-contained facts have been written as if they were one sentence. There is no punctuation, no conjunction, and no signal of any kind between them. In the second, three independent clauses have been run together with nothing between them at all. Both are run-on sentences.

B. Defining a Comma Splice

A comma splice occurs when two independent clauses are joined by a comma alone — without a coordinating conjunction. The comma is not strong enough to join two independent clauses on its own. It signals a pause, not a boundary between two complete thoughts.

Consider the following:

The glacier is retreating, it has lost half its volume since 1900. Charles Darwin sailed to the Galápagos Islands in 1835, the experience changed the course of scientific history.

In the first example, two independent clauses are separated by a comma — but no coordinating conjunction follows. In the second, the same error appears. Both are comma splices.

The comma splice is distinct from the run-on sentence in that it uses a comma — but the comma is insufficient. The run-on sentence uses nothing at all.

C. Why These Errors Occur

Run-on sentences and comma splices typically occur for one of three reasons.

The first reason is that the two ideas are closely related, and the writer feels that a full stop would separate them too abruptly. This is understandable — but a comma alone is not the solution.

The second reason is confusion about the role of conjunctive adverbs such as however, therefore, furthermore, and consequently. Many writers treat these words as if they were coordinating conjunctions and place only a comma before them. They are not conjunctions — they are adverbs — and they require a semicolon before them when joining two independent clauses.

For example:

The data was inconclusive, however the team published their findings.

This is a comma splice. However is a conjunctive adverb, not a coordinating conjunction, and cannot be preceded by a comma alone when joining two independent clauses. The correct punctuation is a semicolon before however and a comma after it.

The data was inconclusive; however, the team published their findings.

The third reason is simple oversight — the writer has not checked where one independent clause ends and the next begins.

D. Four Strategies for Correction

There are four reliable strategies for correcting both run-on sentences and comma splices. Each produces a slightly different result — the choice depends on the relationship between the ideas and the stylistic effect desired.

Strategy 1 — Use a full stop

Separate the two independent clauses into two separate sentences. This is the most straightforward solution and is appropriate when the two ideas are sufficiently distinct to stand alone.

Strategy 2 — Use a semicolon

Join the two independent clauses with a semicolon. This is appropriate when the two ideas are closely related and the writer wants to signal that connection without using a conjunction. The semicolon is stronger than a comma but weaker than a full stop.

Strategy 3 — Use a coordinating conjunction

Join the two independent clauses with a coordinating conjunction — for, and, nor, but, or, yet, so — preceded by a comma. This is appropriate when the logical relationship between the two ideas can be expressed by one of the seven coordinating conjunctions.

Strategy 4 — Use a subordinating conjunction

Rewrite one of the independent clauses as a dependent clause by introducing it with a subordinating conjunction. This is appropriate when one idea is logically subordinate to the other — when one expresses a reason, condition, contrast, or time relationship with respect to the other.

E. Comma Splices with Conjunctive Adverbs

As noted above, conjunctive adverbs are a particularly common source of comma splices. The table below shows the correct punctuation pattern for the most common conjunctive adverbs.

3. Usage in Context

4. Common Errors and Corrections

5. Lesson Mastery

After completing this lesson, you should now be able to:

    ✅ Define a run-on sentence and explain why it is a grammatical error

    ✅ Define a comma splice and distinguish it from a run-on sentence

    ✅ Apply four strategies for correcting run-on sentences and comma splices

    ✅ Recognise and avoid both errors in your own writing