Here we see a sentence with a comma in the middle of it, dividing the sentence into two parts.  But that same thing can be said about many perfectly good sentences, including this one here and the one right before it.  It is only a problem when the two parts are independent clauses.

But if you already knew enough about grammar to understand exactly what an “independent clause” was, you probably would not even be having a problem, would you?  The traditional form of the rule is not all that helpful to most people.  So let me take you through this in two easy stages:  how to find or diagnose comma splices in a draft, and how to fix or correct them.

To find comma splices, first look for commas.  At each comma in turn, imagine changing it to a period (and capitalizing the next word), so as to make two sentences.  If both the parts work out as good sentences and not fragments—that is, if the substitution of a period for that comma works just fine—then you have found a comma splice.  See how it works in our example:

That works just fine.  Both of these are able to stand on their own as complete sentences.  Therefore they must not be joined with just a comma, as in the example up top.

Now try the same test on one of those perfectly good sentences:

Here the first part works as a complete sentence, but the second does not—it is a fragment.  So the original version of this sentence, the version with the comma, passes the comma-splice test.

Now that you know how to find comma splices in your draft, you need to know how to fix them.  There are two main methods, and you can use either one.  Sometimes you can use both together.

The first method is to go ahead and replace the comma with a period.  (Of course you will have to capitalize the word following.)  Or, if you do not want to go quite that far, you can use a semicolon instead, or a dash:

The second method is to add a conjunction, such as and, but, or, so, since, because, for, while, when, where, or [al]though.  Pick one that is appropriate to the relationship (in meaning) between the two could-be sentences that you spliced together:

If we used “so” instead of “since” here the sentence would not make much sense:  the cause-and-effect relationship would be backwards.

Most conjunctions should be added just after the problem comma, as in most recent example.  But occasionally one has to be placed at the beginning of the sentence instead.  For instance, suppose our sentence with the comma splice were in the reverse order:

In this instance, “so” would work quite well inserted after the comma.  “Since” would still work fine too; but if “since” is used, it must come at the beginning of the sentence, not after the comma:

Where the conjunction gets put in right after the comma, you can sometimes combine the two methods, changing the comma to a semicolon or period and also adding the conjunction:

This is most common and graceful with and or but, less so with or, so, or for (as here), and not acceptable at all with the other conjunctions I have listed.

You may have been taught never to start a sentence with And or But, but many experts, including notorious curmudgeon H. W. Fowler (and me), hold that it is perfectly acceptable.  (The King James Bible, a great monument of fine English style, does it all the time:  all but two of the sentences in its famous opening chapter start with “And”!)  One consequence of this is that sentences where and or but follows a comma might seem to fail the comma splice test, since the comma can be replaced with a period without a problem:

This second version may be perfectly acceptable, but in this case the first is too; since you already have an appropriate conjunction following the comma, it is not a comma splice.

Comma splices are acceptable only in rare instances, generally where the clauses are very short and form a kind of matched set of three or more:

An error somewhat less common (and worse) than the comma splice is the fused sentence, which is like a comma splice but without even a comma at the place where a period would or should be:

The best way to find these is to read your work aloud, making sure you do not pause where there is no written punctuation to mark a pause.  If you do this, a fused sentence will sound wrong, and you can proceed to ask yourself whether a period could be inserted anywhere in the sentence without turning either the part before or the part after into a fragment.  The options for correcting this problem are the same as for the comma splice.