A run-on sentence is two or more complete sentences punctuated like a single sentence.
Run-ons make you look either careless about or ignorant of the rules of punctuation.
Types of Run-on Sentences
Fused Sentences run two independent clauses together without any punctuation.
Avid readers enjoy vibrant sex lives but nonreaders tend to frigidity and impotence.
Comma Splices have a comma between the two independent clauses.
The plumber turned off the water, then he repaired the pipes.
(The comma is the 98 lbs. weakling of punctuation marks; it’s not strong enough to hold two independent clauses together by itself.)
How to Correct Run-ons
Run-ons always result from mispunctuation; you can always cure them with proper punctuation. There are four proper ways to punctuate multiple independent clauses:
Period + Capital Letter
Split the run-on into two separate sentences.
Avid readers enjoy vibrant sex lives. Nonreaders tend to frigidity and impotence.
The plumber turned off the water. Then he repaired the pipes.
Semicolon
Join the independent clauses with a semicolon.
Avid readers enjoy vibrant sex lives; nonreaders tend to frigidity and impotence.
The plumber turned off the water; then he repaired the pipes.
(Typically, you use semicolons only to join together closely related sentences.)
Comma + Coordinating Conjunction
Put a comma and a coordinating conjunction between the clauses.
Avid readers enjoy vibrant sex lives, but nonreaders tend to frigidity and impotence.
The plumber turned off the water, and then he repaired the pipes.
English has seven coordinating conjunctions:
*To remember the coordinating conjunctions, think F A N B O Y S.
Subordination
Make one of the independent clauses dependent by putting a subordinating word or phrase in front of it.
Avid readers enjoy vibrant sex lives while nonreaders tend to frigidity and impotence.
After the plumber turned off the water, he repaired the pipes.
Subordination is usually the most elegant and effective way to correct a run-on sentence.