Let’s start with what a sentence IS. Generally speaking, it’s got a subject and a verb and it expresses a complete thought.

“The dog ran.”  Perfect! The subject is dog, the verb is ran, and I’m not left thinking “Huh? What? Who now?”

“On a blustery winter’s day, Mr. Puffenstuff contemplated death.”  Still perfect! The subject is Mr. Puffenstuff, the verb is contemplated, and the only questions you’re probably left with are “What’s blustery, and why was he contemplating death?”

If you want to be super-fancy, you can call that thing (subject-verb-complete thought) an INDEPENDENT CLAUSE.

Run-ons due to comma splice

One problem a lot of us have is creating run-ons by using a comma splice. All that means is that we join two independent clauses with a comma, like this:

The dog ran, Mr. Puffenstuff contemplated death.

That’s a mistake called a comma splice. You’ve spliced (joined) two independent clauses together with a comma. There are three ways to fix that.

First, you can add a conjunction (for, and, nor, but, or, yet, so, although, therefore, etc).

The dog ran, and Mr. Puffenstuff contemplated death.
The dog ran, but Mr. Puffenstuff contemplated death.

Second, you can just make them into two sentences.

The dog ran. Mr. Puffenstuff contemplated death.

Finally, if the two ideas are closely related, you can use a semicolon.  For example, if it had been Mr. Puffenstuff’s responsibility to keep the dog safe, and the dog ran into traffic, you might write this:

The dog ran; Mr. Puffenstuff contemplated death.

Run-ons due to length

Sometimes I see run-on sentences that are just too long. Usually, it’s because you’re trying to fit too many ideas into one sentence. It looks like this:

It was Mr. Puffenstuff’s sole responsibility to care for the dog, who belonged to Miss Gravenstein, a woman from the country club who was known to be both an avid golfer and an animal lover, so when the dog ran into the path of an oncoming bus, Mr. Puffenstuff, in his deep love and admiration for Miss Gravenstein, contemplated death.

Technically, that sentence is grammatically okay. It just contains too many ideas. If there are more than about three separate ideas a reader has to keep track of, you should probably just make a new sentence. Strive for clarity.

Fragments

Most students don’t write sentences like “The dog.” That’s obviously a fragment. Occasionally I do get a fragment that just loses track of its subject, like the following:

In the book The House of the Spirits, by Isabel Allende, teaches us about the power of family.

The problem here is that there’s no subject. Is it supposed to be “In the book… teaches”? Is it “by Isabel Allende teaches”?  A simple fix is to make sure there’s a subject.

The book The House of the Spirits, by Isabel Allende, teaches us about the power of family. [Now the subject is “book.”]

In the book The House of the Spirits, by Isabel Allende, the author teaches us about the power of family. [Now the subject is “author.”]