This sounds like a no-brainer, especially if you've been writing for a long time. You know what constitutes a sentence and what doesn't, right? Maybe not. One of the most frequent errors that I encounter as an editor is a run-on sentence, but it's also one of the easiest to remedy.

A sentence is composed of a noun, a verb, an article, and sometimes an adjective, adverb or preposition. A clause is a part of a sentence; it's only composed of a noun and a predicate (i.e., verb). Thus, the phrase "John Irving lives in New Hampshire" is a clause. It's a complete clause because it has a noun, verb and direct object. However, "John Irving" is a dependent, or incomplete, clause because it's a sentence fragment; it can't stand alone as a complete sentence.

When we're writing, or editing our work, we want to be aware of complete and incomplete clauses. If we have a complete clause, we don't want to combine it with another complete clause by joining the two together with a comma. That's where the difficulty with run-on sentences lies. Often, two thoughts seem so related that we're convinced they belong in the same sentence. For example, "I'm on my way out the door, see you later" is a perfect example of two complete sentences that can't be joined by a comma. Almost any other form of punctuation can take its place. We can use a period, a semicolon or a dash (use an em dash, which is the longer one, not an en dash, which is used to separate numbers.).

If you have a dependent clause, it has to be clarified. We can't form a complete sentence by saying, "After breakfast," because the reader will think immediately, "After breakfast, what? What happened?" Consequently, writers are far less likely to use sentence fragments than they are to run two good sentences together.

That doesn't mean that you can't break the rules, but in order to do so, know them well first. Then you can make all the exceptions you want. Joyce Carol Oates, one of my favorite authors, has an aversion to commas. There are times when she's written entire pages without ever stopping once to insert a period. That's her prerogative. She's a professor of English Lit and knows her stuff. As long as you know when it's appropriate to bring your sentence to a complete halt with a period, or to pause it with a semicolon or dash, you'll be fine.

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