RUN-ON SENTENCE The choice of a camera is difficult, there are many good ones on the market.
These two sentences should either be separated by a period or joined into one sentence by a conjunction or a semicolon. There are four ways of correcting the error:
1. The choice of a camera is difficult. There are many good ones on the market.
2. The choice of a camera is difficult, but there are many good ones on the market.
3. The choice of a camera is difficult because there are many good ones on the market.
4. The choice of a camera is difficult; there are many good ones on the market.
As you grow older and do more and more writing, you develop a "sentence sense," which is the ability to recognize at once whether a group of words is or is not a complete sentence. Reading your compositions aloud, so that your ears as well as your eyes can detect completeness, will help you find any run-on sentences in your own writing.
NOTE Do not be surprised, after being warned against sentence fragments and run-on sentences, if you find them being used occasionally by writers in the best newspapers and magazines. Professional writers (who have a strong sentence sense, or they would not be professionals) do at times write fragments and use the comma between sentences, especially when the ideas in the sentences are very closely related. Leave this use of the comma and the use of the fragment to the experienced judgment of the professional.
Chapter 11 Sentence Completeness, Fragments and Run-on Sentences, in Warriner's English Grammar and Composition Revised Edition, Complete Course, John E. Warriner and Francis Griffith, 1965