Sometimes, as Plato said, it's easier to define something by what it is not rather than what it is. So let's take a dog for example. A domestic animal with four legs, a head and a tail; it responds to love and affection and is fun to have around. Good definition? Well it's ok but it could refer to a cat just as easily. One way to define the dogginess of a dog is to say that it is not a cat.
So what does this have to do with run on sentences? There are almost as many myths, misconceptions and misleading definitions around about run on sentences as there are examples. It might be easier to say what a run on sentence isn't.
A sentence that is long is not necessarily a run on sentence; neither is a sentence with two or more commas. If this really were the definition of a run on sentence, then some of the world's greatest writers would be guilty of this heinous crime. Almost any writer from the Nineteenth Century would fall under this category; imagine telling Dickens that his writing is ungrammatical. Of course, like domestic pets where some are dogs and some are cats, some lengthy sentences are run on. And the following is the reason for this:
A sentence is one complete thought that is comprehensible if heard or read on its own; this is why it is called an independent clause. A run on sentence combines two complete sentences or independent clauses but fails to use a conjunction, punctuation or a transitional word or phrase to join them together.
Punctuation is a tool to aid clarity of meaning in writing. It stands in the place of facial expression, inflexion and tone in spoken language. If punctuation is used correctly then ideas and inferences can be transmitted correctly.
This is not to say that the rules cannot be broken. It means that when the rules are broken, it is deliberate and intended to communicate a particular message. This is true of the last chapter in James Joyce's "Ulysses" where Molly's stream of consciousness is conveyed through the absence of punctuation. No-one is going to accuse Joyce of being ungrammatical; it is his awareness of the rules and subsequent disregard that add so much to that last chapter.
A run on sentence can be as short as four words:
"He pays she stays."
Here we have two independent clauses placed together in a sentence without a link. It is a run on sentence. It seems obvious that the writer wants the reader to make a connection between the two clauses reading the first sentence as the cause and the second the effect. There are a number of options available to this writer to set things straight; the first being to separate this into two sentences.
"He pays. She stays."
Perfectly grammatical sentences, and while they remain in that order the co-dependence of the two scenarios has not been lost entirely. There is an argument for not separating these clauses though because the dependence of one statement on the other is considerably weakened. We are almost reliant now on sentence order to maintain the inferences.
Hemingway is the most famous proponent of short sentences and their efficacy in conveying information and ideas with clarity. I wouldn't want to argue with that but variety is the spice of life. Varying sentence length in a piece of writing should not be merely an attempt to add interest. A sentence's length should be dependent on its purpose. Short sentences are good for describing actions and creating speed of events. Longer sentences are great for descriptions, additional information and building tension and suspense.
Another solution to correcting run on sentences is to divide the clauses with a comma:
He pays, she stays.
Grammar purists would argue that this comma splicing is still ungrammatical. What is required is a co-ordinating conjunction after the comma:
He pays, so she stays.
Or:
He pays, and she stays.
Both these sentences are now grammatically correct but the level of inference has been diminished. So how do we write this sentence grammatically but also retain the layers of meaning? There are three alternatives left to us. The first is to use a semi-colon which grammatically can be used to connect two independent clauses as long as they relate to the same idea:
He pays; she stays.
It is also possible to add a transitional word or phrase after the semi-colon:
He pays; therefore she stays.
The final choice is to use a colon. This works when the second clause refers back to and adds information to the first clause:
He pays: she stays.
Of all the options available in avoiding run on sentences, in this instance I prefer the straight use of the semi-colon showing the two clauses sharing a single idea and the colon which reflects the possible reasons why "she stays". Of course this is my sentence which gives me the liberty to choose the punctuation which best reflects my intended meaning.
Hopefully what this does show is the purpose of correct punctuation in writing and the variety of ways it can help transmit thoughts and ideas with clarity.