When two independent clauses are connected by only a comma, they constitute a run-on sentence that is called a commasplice.
The example just above (about the sunscreen) is a comma-splice. When you use a comma to connect two
independent clauses, it must be accompanied by a little conjunction (and, but, for, nor, yet, or, so).
EXAMPLE: The sun is high, so put on some sunscreen.
Run-on sentences happen typically under the following circumstances*:
a . When an independent clause gives an order or directive based on what was said in the prior independent clause:
This next chapter has a lot of difficult information in it, you should start studying right away.(We could put a period
where that comma is and start a new sentence. A semicolon might also work there.)
b. When two independent clauses are connected by a transitional expression (conjunctive adverb) such as however,
moreover, nevertheless.
Mr. Nguyen has sent his four children to ivy-league colleges, however, he has sacrificed his health working day and
night in that dusty bakery.(Again, where that first comma appears, we could have used either a period — and started
a new sentence — or a semicolon.)
c. When the second of two independent clauses contains a pronoun that connects it to the first independent clause.
This computer doesn't make sense to me, it came without a manual. (Although these two clauses are quite brief, and the
ideas are closely related, this is a run-on sentence. We need a period or a semicolon where that comma now stands.)
Most of those computers in the Learning Assistance Center are broken already, this proves my point about American
computer manufacturers. Again, two nicely related clauses, incorrectly connected — a run-on. Use a period or semicolon
to cure this sentence.
The above are all names given to compound sentences that are not punctuated correctly. The best way to avoid such
errors is to punctuate compound sentences correctly by using one or the other of these rules.
1. Join the two independent clauses with one of the coordinating conjunctions (and, but, for, or, nor, so, yet), and use a
comma before the connecting word.
_________________________, and _________________________.
(He enjoys walking through the country, and he often goes backpacking on his vacations.)
2. When you do not have a connecting word (or when you use a connecting word other than and, but, for, or nor, so, or yet
between the two independent clauses) use a semicolon (

.
__________________________;_______________________ ______.
(He often watched TV when there were only reruns; she preferred to read instead.)
or
__________________________; however,____________________.
(He often watched TV when there were only reruns; however, she preferred to read instead.)
So, run-ons and fused sentences are terms describing two independent clauses which are joined together with no
connecting word or punctuation to separate the clauses.
Incorrect: They weren't dangerous criminals they were detectives in disguise.
Correct: They weren't dangerous criminals; they were detectives in disguise.
Incorrect: I didn't know which job I wanted I was too confused to decide.
Correct: I didn't know which job I wanted because I was too confused to decide.