A run-on sentence is composed of two or more independent clauses. An independent clause is a part of a sentence that can stand on its own. It is not the length of the sentence that makes it a run-on sentence but rather a structural flaw.
When two independent clauses are connected by only a comma, they form a comma-splice. When you use a comma to connect two independent clauses, it must be accompanied by a conjunction (and, but, for, nor, yet, or, so).
Run-on sentences may happen under the following conditions:
a. When an independent clause gives an order or directive based on what was said in the prior independent clause.
b. When two independent clauses are connected by a transitional expression (conjunctive adverb) such as however, moreover, nevertheless.
c. When the second of two independent clauses contains a pronoun that connects it to the first independent clause.
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ESL Study Guide