Run-On Sentences
Let's check out run-ons. A run-on sentence, or fused sentence, is a sentence that's missing the right punctuation to make it flow properly. A run-on occurs when what could be two complete sentences - that is, two independent clauses - are connected in one sentence without being punctuated. In other words, they're fused together instead of each clause being distinguished from the other. Here's an example of a run-on:
Clark had vanished he left his eyeglasses and coat in the telephone booth.
You have two independent clauses here, Clark had vanished and He left his eyeglasses and coat in the telephone booth. To make this a proper sentence and not a run-on, you must add proper punctuation. You have several different options - which you choose is a matter of your personal style and how you want to make the sentence look and feel to your reader.
You can add a comma and a coordinating conjunction. You can remember coordinating conjunctions by the mnemonic FANBOYS - that's for, and, nor, but, or, yet, and so. So:
Clark had vanished, but he left his eyeglasses and coat in the telephone booth.
You can make the run-on into two separate sentences:
Clark had vanished. He left his eyeglasses and coat in the telephone booth.
Or you can add a semicolon:
Clark had vanished; however, he left his eyeglasses and coat in the telephone booth.
The choice is yours, and it doesn't really matter which you choose as long as you've fixed the run-on.
One other thing to remember is that even though we tend to think of run-ons as sentences that are too long, a run-on is really defined by improper punctuation. I sang I danced. - despite being just four words - is a run-on sentence.
Comma Splices
Technically, a comma splice is a kind of run-on sentence, but they're harder to spot, so we're giving them their own section. A comma splice occurs any time a comma is inserted between two main clauses without a coordinating conjunction to connect them. For instance:
Most of us liked to eat fish, Jerry preferred to devour the still-beating hearts of fresh artichokes.
A sentence like this sounds fine when you read it out loud and it even looks pretty okay when you see it on the page, but it's wrong. If you're going to use a comma to separate these two independent clauses, you need to insert coordinating conjunction (that's FANBOYS like we talked about a little bit earlier) after the comma, making the sentence, for instance, :
Most of us liked to eat fish, but Jerry preferred to devour the still-beating hearts of fresh artichokes.
Because a comma splice occurs between two independent clauses, you can also remove the comma and replace it with a period (making two separate sentences) or a semicolon, like so:
Most of us liked to eat fish; Jerry preferred to devour the still-beating hearts of fresh artichokes.
The most basic thing to remember about comma splices is that they're really just a misused comma where other punctuation would be more appropriate.