Your proofreader has scrawled a note on your manuscript that says, "This isn't a sentence."
You look down and consider that the "sentence" in question has a capital letter at the beginning and punctuation at the end. Isn't that enough? Actually, it isn't.
When writing for young children, there are editors who are touchy about the use of complete sentences. This is because they want your writing to be a good influence on those impressionable youngsters who are still in the process of learning the English language, rather than annihilating it like the rest of us.
As you write for older children, you're allowed a little more leeway. Dialogue, in particular, sounds stilted when it's written as complete sentences. This is because no one really talks that way except for aliens from other planets who, if you've noticed, never use contractions.
So what is a sentence?
A sentence must meet two requirements.
1. It must contain a clause.
2. It must sound complete.
In case you didn't know, a clause is made up of at least a subject and a verb. Independent clauses sound complete, and therefore are sentences. Here are a few examples.
Bob threw the ball.
Mary is friendly.
Jennifer is running.
By comparison, here are some dependent clauses. You'll notice that they don't sound complete, even though they each contain a subject and a verb.
If Bob threw the ball.
When Mary is friendly.
Where Jennifer is running.
These could be fixed by attaching them to an independent clause or by rewriting them.
If Bob threw the ball, he's in big trouble.
When Mary is friendly, she doesn't eat your Cheetos.
There is a vicious bull in the field where Jennifer is running.
In short, a sentence has to have a subject and a verb, and it has to sound complete. If it doesn't sound complete, it must be fixed before the grammar police nab you.
If you have any questions, please leave a comment and I'll get to them in future posts.