Run-on sentences occur when two or more independent clauses are joined without using a coordinating conjunction (e.g., for, and, nor, but, or, yet, so) or correct punctuation (e.g., commas, semicolons, dashes, or periods).
A run-on sentence can be as short as four words.
I drive she walks.
In this case, there are two subjects (I, she) paired with two intransitive verbs (drive, walks).
An imperative sentence can be a run-on even if it only has two words.
However, with correct punctuation, a writer can assemble multiple independent clauses in a single sentence. A properly constructed sentence can be extended indefinitely.
It is important to realize that the length of a sentence has nothing to do with whether a sentence is a run-on or not. A run-on sentence is a structural flaw that can affect even a very short sentence.
The sun is high, put on some sunblock.
When two independent clauses are connected by only a comma, they constitute a run-on sentence that is called a comma-splice. The example above is a comma-splice. When a comma is used to connect two independent clauses, it must be accompanied by a little conjunction (and, but, for, nor, yet, or, so).
For example, the following sentence is correct:
The sun is high, so put on some sunscreen.
Run-on sentences happen typically under the following circumstances:
1. An independent clause gives an order or directive based on what was said in the prior independent clause.
This next chapter has a lot of difficult information in it, you should start studying right away.
We could put a period where that comma is and start a new sentence. A semicolon would also work.
2. Two independent clauses are connected by a transitional expression (conjunctive adverb) such as however, moreover, or nevertheless.
Mr. Nguyen has sent his four children to ivy-league colleges, however, he has sacrificed his health working day and night in that dusty bakery.
Again, where that first comma appears, we could have used either a period — and started a new sentence — or a semicolon.
3. The second of two independent clauses contains a pronoun that connects it to the first independent clause.
This computer doesn’t make sense to me, it came without a manual.
Although these two clauses are quite brief, and the ideas are closely related, this is a run-on sentence. We need a period where that comma now stands.
Most of those computers in the Learning Assistance Center are broken already, this proves my point about American computer manufacturers.
Again, two nicely related clauses, incorrectly connected — a run-on. Use a period to cure this sentence.