A run-on sentence is not simply a sentence that is too long. Rather, it is a sentence in which two or more independent clauses are fused together without the proper punctuation or conjunctions needed to hold them together in a grammatically correct way. There are many types of run-ons. We’ll cover the three most common.
1. Comma splices
I need a new TV, mine’s broken.
The movie was good, the special effects were awesome.
These could be corrected in a number of ways. The two clauses could be made separate sentences:
I need a new TV. Mine’s broken.
The writer could replace the comma with a semicolon:
The movie was good; the special effects were awesome.
An em-dash can do the same thing:
I need a new TV—mine’s broken.
With a subordinating conjunction, one clause could become dependent on the other:
Because the special effects were awesome, the movie was good.
Or the writer could link them with a coordinating conjunction:
My TV’s broken, so I need a new one.
2. Two sentences crammed together
Other run-on sentences consist of two independent clauses crammed together with no punctuation—for example:
Let’s go it’s getting hot.
All the comma splice solutions apply here as well:
Let’s go. It’s getting hot.
Let’s go; it’s getting hot.
Let’s go—it’s getting hot.
Since it’s getting hot, let’s go.
It’s getting hot, so let’s go.
3. Conjunction misuse
Another common type of run-on sentence involves misuse of conjunctions, especially however, often with a comma splice—for example:
I have kits to assemble, however, I have no bags.
Here, there’s a comma splice after I have kits to assemble. This common error probably results from confusion with perfectly acceptable constructions like this:
Over many years of jurisprudence, however, the Supreme Court has ruled that most of the rights protected in the Bill of Rights also apply to the states. [SCOTUS Blog]
This however works because it functions adverbially (here it is synonymous with though). The however in the first example is a coordinating conjunction, so its clause should be treated as independent.