Run on sentences are sentences that contain too many ideas without proper punctuation. Not all long sentences are run on sentences. It is perfectly acceptable to join several related ideas in one compound sentence. However, if you don't follow punctuation rules, a sentence can become a run on. A simple explanation of run ons and some examples of run on sentences should help to make this point clear.
Each sentence has 3 necessary components
For example, lets look at the sentence "Jim is cold." This sentence has a subject: "Jim." It has an action: "is" which is a form of the "to be" verb. Jim is doing the action of "being cold." It also expresses a complete thought- the purpose of this sentence is to tell the reader that Jim is cold.
Sentences which have these 3 components are called Independent Clauses. If a sentence is lacking in one of the 3 components, it is called a Dependent Clause. Independent clauses can stand on their own- they form their own sentences. Dependent clauses can't stand alone- they need to be joined to another clause.
A run on sentence is a sentence that has 2 or more clauses which are improperly joined. Independent and dependent clauses can be joined together to create compound sentences when writing.
However, there are rules that have to be followed when creating compound sentences, in order to avoid creating a run on sentence:
Related independent clauses are clauses that could stand on their own, but that are related to each other. For example: "Jim is cold; he wants to go inside." Each of the two clauses in this sentence are independent clauses- they could stand by themselves. They are related, so they are joined together by a semicolon.
An independent clause can be made dependent on another clause, by using a subordinating conjunction. For example: "Jim is cold but he wants to say outside." The second clause "He wants to stay outside." could be an independent clause. However, the subordinating conjunction "and" makes it dependent on the first clause. With the "and" at the beginning, the sentence could no longer stand on its own. It is dependent on the first clause.
The following 3 examples of run-on sentences demonstrate problems that occur when compound sentences are formed without proper punctuation:
"Kelly likes to cook" is an independent clause that could stand by itself. "She makes chicken every day" is an independent clause that could stand by itself. These two sentences cannot be joined by a comma. The improper use of a comma here is called a comma splice. The sentence could be corrected by using a semicolon in place of the comma. It could also be corrected if a conjunction was used: "Kelly likes to cook, and she makes chicken every day."
2. Mary likes dogs she has a beagle.
"Mary has dogs" is an independent clause that could stand by itself. "She has a beagle" is an independent clause that could stand by itself. As written, the sentence is a run on sentence since it contains two separate ideas. These two clauses must have punctuation and/or a conjunction.
3. Ann likes to read; she reads lots of books; she goes to the library 3 times a week.
While semicolons can be used to link independent thoughts, they can only be used to link 2 independent thoughts. You can't use 2 semicolons to link 3 independent thoughts. One of these thoughts needs to be a separate sentence, or a conjunction needs to be used. For example, it would be acceptable to say "Ann likes to read; she reads lots of books and goes to the library 3 times a week."