A run-on sentence is a sentence with at least two independent clauses (complete thoughts), which are forced together instead of being properly connected.
To correct a run on, you have several options:
1) Separate clauses using punctuation (semicolon between independent clauses, a dash if you want to show a break, a colon if the second part explains the first)
2) Separate clauses using a comma and a coordinating conjunction (For, And, Nor, But, Or, Yet, So)
3) Rearrange the sentence (you may add or remove words, make one clause dependent on another).
4) Separate clauses with a semicolon and a conjunctive adverb (however, furthermore, otherwise, moreover, thus, etc.)
He put on some sunscreen, the sun was so extremely hot, and he wanted to go inside.
“He put on some sunscreen” 1st clause
“the sun was so extremely hot” 2nd clause
“he wanted to go inside” 3rd clause
He put on some sunscreen because the sun was extremely hot. He wanted to go inside.
Examples of Sentences Punctuated Correctly
The dark skies and distant thunder dissuaded Clarice from her afternoon run; moreover, she had thirty calculus problems to solve for her morning class.
Leon's apartment complex does not allow dogs over thirty pounds; otherwise, he would have bought the gangly Great Dane puppy playing in the pet store window.
I didn't think it would be necessary to take my umbrella; after all, the sun was shining when I left home. (semicolon and a conjunctive adverb and then a comma)
A fifth-grade student from our school won the spelling competition; she spelled words I had never heard before. (semicolon)
The storm passed quickly through the area last night, and it caused a lot of damage. (a comma and a coordinating conjunction)
I've missed several classes because I was ill. (subordinate one clause by adding a subordinating conjunction such as BECAUSE, SINCE, WHEN, AFTER, ALTHOUGH)
Check out Purdue's Overview of Run Ons
Check out https://owl.purdue.edu/owl/general_writing/punctuation/independent_and_dependent_clauses/runonsentences.html for more information
Review Handout on Comma Splices (a type of run on)
Review Handout on Fused Sentences (another type of run on)
A fragment is an incomplete thought. A run-on is a sentence that doesn’t use proper punctuation to connect independent clauses. Complete sentences have a subject and a verb and are a complete thought.
FRAGMENT: The sun shining, the birds chirping, signaling a new day.
Complete sentence: The sun shining, the birds chirped, signaling a new day.
FRAGMENT: The music playing in my ears, the crowd cheering (not a complete thought with a subject and verb)
COMPLETE SENTENCE: The music played in my ears as the crowd cheered.
Check out Purdue's Overview of Fragments
Check out https://owl.purdue.edu/owl/general_writing/mechanics/sentence_fragments.html for more information
Review Handout on Fragments