Three Errors with Compound Sentences

You have two connected ideas, two ideas that together make a complete message. In fact, the connection is so close that you want to join them into one long sentence. For example, let's say you want to communicate these two ideas:

1. The man entered with a gun.
2. Bank personnel were not amused.

You want to write them in one longer sentence, so you join them together. This is where it gets complicated. Mistakes are common. Care is needed.

First Error: Comma Splice

Because the ideas are closely connected together and form one broader idea, you might join them like this:
*The man entered with a gun, bank personnel were not amused.

Whoops! You just made a comma splice. And that's an error.

Comma Splice: Joining two independent clauses with only a comma

An independent clause has a subject and a predicate, and it can serve as a complete sentence. In our sample, the first independent clause is The man entered with a gun. That could be a complete sentence. The second independent clause is bank personnel were not amused. That, too, could be a complete sentence.

As we see in this example, you joined the two independent clauses with only a comma, creating a comma splice.

Second Error: Fused Sentence

On the other hand, you might simply join the two ideas without anything to connect them, as follows:
*The man entered with a gun bank personnel were not amused.

Whoops! You just created a fused sentence. And that's an error.

Fused Sentence: Joining two independent clauses without any type of connector

The second type of error is not a comma error. Rather, it is an error of omission. Here, you have the same two independent clauses, but you have not put anything between them to join them. You fused them together without a connector.

Third Error: Run-on Sentence

Even if you avoid the first two types of errors, you still have to watch out for this one. You know that two independent clauses can be joined with a coordinating conjunction (i.e., for, and, nor, but, or, yet, so). To show how your two ideas are related, you might join them with just the conjunction, like this:
*The man entered with a gun and bank personnel were not amused.

Whoops! You just created a run-on sentence. And that's an error.

Run-on Sentence: Joining two independent clauses without the appropriate punctuation

Look at the sentence construction. Again, you have the same two independent clauses, and you joined them with a coordinating conjunction, thus avoiding the first two errors. However, that conjunction is all by itself, violating Zen Comma Rule D: Put a comma before a conjunction that joins two independent clauses.

To understand why this is wrong, think about what a comma does. The central purpose of the comma is to indicate parts of a sentence that have a specific meaning. In this sample, we have two major parts, each of which has a specific meaning. We need a comma to indicate where the first meaning ends and the second one begins. In this sample, you left out that comma.

Writers sometimes do this to show how closely the ideas are connected, but it's an error and must be used infrequently and with great care. If you're going to make a mistake, such as this one, do it on purpose—not accidentally.

Solutions to the Errors

The solutions to all three errors are the same. If you want to join two independent clauses in a single sentence, you have only three options.

1. Comma + conjunction: The man entered with a gun, and bank personnel were not amused.
2. Semicolon: The man entered with a gun; bank personnel were not amused.
3. Colon: The man entered with a gun: bank personnel were not amused.

Although each of these is correct, I would choose the second option. It has more oomph than the first option because it doesn't have the conjunction to soften the connection and further separate the ideas. The third option with the colon is not as common as in prior years and may be a little distracting without adding extra value. Whichever option you choose to create a compound sentence, the sentence will be correct.