A common mistake with commas (the run-on error)
 
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.
 

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 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.)

Run-on Error

Once you have formed a sentence (i.e., expressed a complete concept), you must put a full stop (or 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 in this case) and carry on writing.)

Your Idea Could Consist of Several Sentences

Writers often feel that a comma is more appropriate than a full stop (or period ) because their sentences are so closely related.  In other words, they sense that a full stop (or period ) is too abrupt, because they haven't finished expressing their idea.  This is at the root of this error. 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.
(It may be possible to put a comma and a conjunction (and in this example) and carry on writing. This is not an error.)

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

The truth is incontrovertible, malice may attack it, ignorance may deride it, but in the end; there it is. (Winston Churchill)
(The semicolon in this example is also wrong.)

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)

 
ALTERNATIVES TO A COMMA

Occasionally, it may be appropriate to use a dash or a semicolon instead of a full stop. See the lesson Extend a Sentence.
 

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)


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