If you write “And in the morning” and put a period after what you have written, the sentence has been left incomplete. It’s a sentence fragment, not a complete sentence. Your reader will be left wondering “And in the morning, what?” In the other direction, if you write “The day was bright and sunny I got up early it was my birthday” and put a period after what you have written, you have written three complete ideas, not one. What should be three sentences has been written as a single run-on sentence. That’s a grammatical error, and it’s not one that can be fixed by “splicing” the three parts together with commas (“The day was bright and sunny, I got up early, it was my birthday.”).
A run-on sentence is a sentence that continues running on and on when, as a matter of grammatical correctness, it should be broken up into two or more sentences. Sometimes people use the expression run-on sentence loosely to refer to a sentence that is simply very long, regardless of whether or not it is grammatically correct. A well-constructed long sentence, though, can be an excellent means of expressing complex ideas; for the sake of clarity, then, it’s important not to confuse the idea of a long sentence with the idea of a run-on sentence. The term run-on sentence should be used only when issues of grammatical correctness are involved.
One variety of run-on sentence (a fused sentence) occurs when independent clauses are not separated by any punctuation. A second (and more common) variety of run-on sentence occurs when a comma (rather than a period or a semicolon) is used between independent clauses, without the addition of any coordinating conjunction. This type of run-on sentence is called a comma splice.
In simple examples such as those above the matter may seem straightforward. Sometimes, though, it is not so simple. The conventions of English dictate that only certain words may be used to join two independent clauses into one sentence; the seven coordinating conjunctions (and, but, or, for, nor, so, and yet) may be used to join independent clauses into one sentence; words such as therefore, hence, otherwise, and then may not be used in this way.
The bottom line is that run-ons occur when two or more independent clauses are joined improperly, often with a comma, or with nothing. The main ways to fix a run-on are to separate independent clauses with a period, to separate them with a semicolon, or to revise (usually with the help of a coordinating conjunction) so as to make one clause into a dependent clause.
More on run-on sentences and sentence fragments may be found in The Broadview Guide to Writing under “Punctuation Marks: The Period,” and “The Comma” in The Broadview Pocket Guide to Writing under “Run-On Sentences.” If you are unclear on the meanings of subject and predicate, clause and phrase, or main clause and subordinate clause, see section M1.3, “Parts of Sentences.”
Analyze and identify the sentence status of each of the following examples.