1. (Also called Run-on Sentences)
Fused Sentences
(Also called Run-on Sentences)

2. A fused sentence occurs when two independent/main clauses are joined (fused) without any punctuation.
An independent/main clause is a complete sentence (has a subject, verb, and complete thought)

3. Example of a fused sentence:
Driving home from school, Brett vowed to protect the fragile ecosystem all the while the tires of his Cadillac Escalade flattened the toads hopping on the wet streets.
The first main clause is Brett vowed to protect the fragile ecosystem
and the second is the tires of his Cadillac Escalade flattened the toads hopping on the wet streets.
Notice that the two clauses run together with no punctuation.

4. How to fix a fused sentence:
Option 1: Period + Capital Letter
First, you can break the error into two separate sentences, like this:
Driving home from school, Brett vowed to protect the fragile ecosystem. All the while, the tires of his Cadillac Escalade flattened the toads hopping on the wet streets.

5. Option 2: Comma + Coordinating Conjunction (FANBOYS)
Another good option is to connect the two main clauses with a comma and a coordinating conjunction:
Driving home from school, Brett vowed to protect the fragile ecosystem, yet all the while, the tires of his Cadillac Escalade flattened the toads hopping on the wet streets.

6. Option 3: Semicolon
You can also use a semicolon, a mark of punctuation as strong as a period:
Driving home from school, Brett vowed to protect the fragile ecosystem; all the while, the tires of his Cadillac Escalade flattened the toads hopping on the wet streets.

7. Option 4: Subordination
Your last option is to use a subordinate conjunction. This method reduces one of the two clauses to an incomplete thought:

8. Subordination: To use a subordinate conjunction, you will have to change the sentence—keep the first independent/main clause as it is and adjust the second main/independent clause to make it a dependent clause.
Driving home from school, Brett vowed to protect the fragile ecosystem as the tires of his Cadillac Escalade flattened the toads hopping on the wet streets.