A run-on is when two sentences are "combined," or "run together" without the necessary connecting words or punctuation (or both of these!).
This is a fused sentence because there is no period between the two sentences.
If we add a period, we get two complete sentences.
This is a comma splice sentence.
If we add a connecting word, we get one complete sentence.
1. Put a period between the two sentences.
This is a run-on.
Now these are two good sentences.
2. Connect the two sentences with a comma and a coordinating conjunction (and, but, nor, for, so, or, yet). This forms a compound sentence.
This is a run-on.
This is a good sentence.
3. Connect the two sentences with a semicolon.
This is a run-on.
This is a good sentence.
4. Connect the two sentences with a semicolon and a word or phrase to explain the relationship between the two sentences (however, therefore, then, as a result, consequently, also, in contrast, nevertheless, on the other hand, etc.).
This is a run-on.
This is a good sentence.
5. Connect the two sentences by inserting one as a dependent clause. You will often use a dependent clause signal word to do this (because, if, although, which, who, when). This forms a complex sentence.
This is a run-on.
This is a good sentence.