The Run-on Error (A Common Mistake with Commas)
 
Once you have written a sentence, do not be tempted to put a comma and write another sentence.  This is the most common mistake involving commas.  It is called a run-on error.
 

(Before We Start with Run-On Error) What Is a Sentence?

A sentence is a grammatically complete series of words.

A sentence must have a subject and a verb, even if one or the other is implied, and it must express a complete concept. A sentence begins with a capital letter and normally ends with a full stop (period ).

(NB: A sentence can also end in a question mark (?) or an exclamation mark (!). If we're being really pedantic, it could also end in a speech mark, but it definitely can't end in a comma.)

What Is a Run-on Error?

Once you have formed a sentence (i.e., expressed a complete concept), you must put a full stop / period or another valid ending (like ! or ?) and end the sentence. Do not insert a comma and continue writing. This is a very common mistake. It is known as a run-on error, a run-on comma, or a run-on sentence. For example:

  • Pick up a copy of our free brochure, this explains how to contact us and reach our showroom.
  • (This is two sentences. You cannot put a comma after brochure and carry on writing.)

  • Everyone is aware of the road works in the village, we are still here, come and visit us.
  • (This is three sentences. You cannot put a comma after village and here and carry on writing.)

Your Idea Could Consist of Several Sentences

Writers often feel that a comma is more appropriate than a full stop / period because their sentences are so closely related. In other words, they sense that a full stop / period is too abrupt because they haven't finished expressing their idea.

Remember, a sentence is a grammatically complete series of words. Often, it will take several sentences to complete your idea. Look at the examples below:

  • I always pass on good advice. It is the only thing to do with it. It is never of any use to oneself. (Oscar Wilde)

  • It's not true that I had nothing on. I had the radio on. (Marilyn Monroe)

  • Only one man in a thousand is a leader of men, the other 999 follow women. (Groucho Marx)

  • Be kind to those that meet you as you rise, you may pass them again as you fall.

  • Drink is the curse of the land. It makes you fight with your neighbor. It makes you shoot at your landlord, and it makes you miss him.
  • (Of course, it is possible to put a comma and a conjunction (and in this example) and carry on writing. This is not an error. It is extremely common.)

  • Age is not a particularly interesting subject. Anyone can get old. All you have to do is live long enough. (Groucho Marx)

  • This suspense is terrible. I hope it will last. (Oscar Wilde)

  • Duty is what one expects from others, it is not what one does oneself. (Oscar Wilde)

  • Please don't eat me! I have a wife and kids. Eat them! (Homer Simpson)

  • Please do not shoot the pianist. He is doing his best. (Oscar Wilde)

  • When will I learn? The answer to life's problems aren't at the bottom of a bottle, they're on TV. (Homer Simpson)

 
EXTENDING A SENTENCE WITH A SEMICOLON, A DASH, OR THREE DOTS

Run-on errors occur because writers feel that their ideas need to be crammed into single sentences. They don't. Occasionally, however, it may be appropriate to use a dash, a semicolon, or three dots to extend a sentence.

Read more about extending sentence with a dash, a semicolon, or three dots.
 

See also:

What are conjunctions?
Commas before conjunctions
Commas after a sentence introductions
Commas after a transitional phrase
Commas after interjections (yes, no, indeed)
Commas for parenthesis
Commas in lists
Commas with a long subject
Commas with numbers
Commas with quotation (speech) marks
Commas with the vocative case
Commas with Dear, Hello, and Hi
Semicolons to extend a sentence
Extend a Sentence (dashes, semicolons and three dots)