Run-on sentences are like the person at the all-youcan-eat buffet who overfills a plate when he or she could have simply gone back for a second helping. Run-on sentences are two or more independent clauses written as though they were one sentence. The main cause of run-on sentences, like fragments, is faulty punctuation. End marks like periods, exclamation points, and question marks (Lesson 13) can make or break a sentence. 

Example 1 This run-on sentence is missing punctuation: 

RUN-ON: Julie studies hard she is trying to win a fellowship next year.

CORRECT: Julie studies hard. She is trying to win a fellowship next year.

Example 2 

RUN-ON: The soccer game ended at four, it was too late to go to the birthday party. 

CORRECT: The soccer game ended at four; it was too late to go to the birthday party. 

Commas, when used with a conjunction, can transform run-on sentences. Conjunctions come in three types: coordinating, correlative, and subordinating. Coordinating conjunctions (and, but, or, nor, so, for, yet) can be used to correct run-on sentences. 

Example 3 

RUN-ON: Gillian lived in Portland she lived in New York. 

CORRECT: Gillian lived in Portland, and she lived in New York


Source: p.26-27 Smith, Brady. Proofreading, revising, and editing skills : success in 20 minutes a day / Brady Smith.—1st ed.