We know that a simple sentence is one complete thought. A period should go at the end of the complete thought.
When a writer uses a comma between two complete thoughts, we end up with a run-on sentence.
Here are two complete thoughts written as one sentence with a comma joining them:
I’m hungry, let’s get some lunch.
This grammar mistake is called a comma splice. A comma splice results in a run-on sentence.
The two thoughts are related – you’re hungry and you want to eat – but they are two complete and separate thoughts.
Use a period after each complete thought. It should look like this:
I’m hungry. Let’s get some lunch.
Always use a period between two simple sentences. Each simple sentence is a complete thought. End each complete thought with a period. Avoid the comma splice.