Here are some tips to help you recognize and avoid using run-on sentences in your writing.

Consider the following:
A sentence can become a run-on sentence just because you forget to put in a comma. For example: The current was swift and he could not swim to shore.
(This sentence is a run on because there is not a comma in front of the short conjunction that joins the two sentences.)

2. It is possible you might accidentally use "only" a comma to join two sentences which creates a comma splice, and a run-on sentence. For example: The current was swift, he could not swim to shore.
(This sentence is a run on because you cannot join two complete sentences with just a comma.)

3. Sometimes you might join two complete sentences without any punctuation at all. For example:
The current was swift he could not swim to shore.
(This sentence is a run on because there is no punctuation or conjunction between the two complete sentences.

The easiest way to avoid run on sentences is to scan the sentences you have written when you do your proofreading. Look for subjects and verbs. Determine if you have written two sentences. If you have, be sure you have joined them in one of the following ways.

(1)You can join two sentences with a coordinating conjunction:
,and ,for
,but ,nor
,or ,yet

(Be absolutely sure you place a comma in front of these short conjunctions.) For example:
The current was swift, and he could not swim to shore.

(2) You can join two sentences with a semicolon (;)
(Be sure the sentences are fairly short and closely related.
The current was swift; he could not swim to shore.

(3)You can join two sentences with a longer conjunction such as therefore, moreover, nevertheless, consequently, etc. If you use a longer conjunction, you must place a semicolon in front of it and a comma after it.
For example: The current was swift; consequently, he could not swim to shore.

If you are careful to determine that you have written two sentences, and then join them together with the proper punctuation, the problem of using run-on sentences in your writing will disappear.