When you join two or more independent clauses without a conjunction or proper punctuation, you’ve created a run-on sentence. It’s a grammatical mistake that can cause confusion or misunderstanding.
Here is a simple run-on sentence:
Julie went to the store she took the car.
See what I’ve done? I’ve smashed together two independent clauses, each which could stand on its own, to make one run-on sentence. Some writers remedy this with a comma, creating another grammatical mistake: the comma splice. That looks like this:
Julie went to the store, she took the car.
One way to fix a run-on sentence is with a conjunction:
Julie went to the store and she took the car.
You can also create two sentences:
Julie went to the store. She took the car.
When the clauses are closely related, you can use a semicolon:
Julie insisted on going to the store herself; she is still afraid of trusting others.
You can also use a semicolon and a transitional expression:
Julie stayed home from work; however, she’s still not recovering.
Which option you choose depends upon the demands of the sentence. Make sure to choose the one that lets the sentence flow smoothly.
Lastly, don’t confuse long sentences with run-on sentences. Even very long sentences can avoid run-on status with the correct punctuation and conjunctions. Long sentences can mirror the thought process or create a sense of urgency and anticipation. Russell Banks and Andre Dubus are two writers who use grammatically correct long sentences to great effect.