Never ever separate two sentences with a comma. Learn what to do instead.
The Short Story
What is a comma splice? It’s when a comma separates two complete sentences.
Incorrect: My cat likes only one kind of food, when it’s on sale, I always buy a few bags.
Correct: My cat likes only one kind of food. When it’s on sale, I always buy a few bags.
The Novel
This rule is pretty straightforward but is one that many people don’t know about. If you’re going to learn any rule about punctuation, remember this one. Some other comma rules you can get away with not knowing—but not this one.
Most of us can recognize a complete sentence. A complete sentence has a subject and a predicate. The subject is what the sentence is talking about. The predicate has a verb and states something about the subject.
Let’s look at an example:
Ex: The fierce dragon blew a stream of fire at the cluster of knights.
What are we talking about? The fierce dragon. That’s our subject.
What did the dragon do? Blew a stream of fire at the cluster of knights. That’s our predicate.
Ex: The fierce dragon blew a stream of fire at the cluster of knights.
Simple, right?
Let’s look at another example:
Ex: The knights were prepared, a wizard had warned them about the dragon.
What are we talking about? The knights. Right. There’s our subject.
What did the knights do? They were prepared. Right again. That’s our predicate.
So, what about the rest of the sentence? Well, it looks like this sentence has another subject and predicate.
What else are we talking about? A wizard. There’s our second subject.
What did the wizard do? Had warned them about the dragon. There’s our second predicate.
Ex: The knights were prepared, a wizard had warned them about the dragon.
Here, we see two subjects and two predicates. They are separated by a comma. This is a comma splice! This is an incorrect way to use a comma. So how can we fix it?
Here are three ways we can fix this comma splice:
1. Use a semicolon.
Ex: The knights were prepared; a wizard had warned them about the dragon.
2. Add a connecting word.
Ex: The knights were prepared because a wizard had warned them about the dragon.
3. Use ending punctuation.
Ex: The knights were prepared. A wizard had warned them about the dragon.
The Examples
“I always read the reviews when I’m thinking about buying something, that way, I know if it’s worth it to spend the money.”
Is this a comma splice? Yes. There are two complete sentences here.
I always read the reviews when I’m thinking about buying something.
That way, I know if it’s worth it to spend the money.
Note: “That way” is an introductory element that isn’t part of the subject or predicate.
“Sheila passed me a slice of thick chocolate cake, and I wolfed it down in two bites.”
Is this a comma splice? Nope. Even though there are two subjects and two predicates, this sentence still counts as one complete sentence and not two because it is linked by the conjunction “and.”
Sheila passed me a slice of thick chocolate cake, and I wolfed it down in two bites.
“Tangled vines and thick branches covered the forest floor, they blocked the secret entrance.”
Is this a comma splice? Yes. We have two complete sentences separated by a comma.
Tangled vines and thick branches covered the forest floor.
They blocked the secret entrance.
Because there is nothing linking the two sentences, each sentence is complete and should be separated by a period or other ending punctuation, not by a comma.