Grammarians, how do I quit writing run-on sentences? Some part of my mind keeps rebelling against the rules.

Sadly, most folks answering this question haven’t the dimmest notion how to define a run-on sentence, let alone how to avoid writing one. So let’s start by shedding light on what a run-on sentence is and is not.

Disproving Some Common Bad Advice

Here are two things to remember about run-on sentences:

A run-on sentence does not necessarily run too long or combine too many ideas.

A run-on sentence does combine two or more Independent clauses without using an appropriate conjunction.

The key distinction to observe is run-on sentences contain a grammatical error, whereas overlong sentences contain a stylistic flaw. To avoid run-on sentences, therefore, one must have a rudimentary understanding of English grammar. And it’s helpful to have a passing familiarity with style, as well, so as not to be misled by questions of length.

To drive this point home, here are a few examples of run-on sentences that all happen to be quite short. Note that each is obviously ungrammatical to any native English speaker.

The boys ran dogs chased them.

They deserve they bite.

They took you grew.

Farms prosper apples grow.

And now, for good measure, let’s combine them into one longer sentence that includes all three run-on sentences without reproducing the errors.

The boys ran because dogs chased them, though they deserve to be bitten since they took what you grew, and farms only prosper when apples grow.

In this example, making the sentence longer actually resolved the grammatical error. Tant pis for the advice to keep it short and simple.

A Better Rule to Follow

The actual rule to follow is this:

Find each predicate and its subject, then rewrite your sentence in such a way the relationships between all predicates is clear.

You may recall from grade-school grammar lessons a predicate has something to do with verbs. But predicates and verbs actually belong to logically distinct categories, because a verb is merely a part of speech whereas a predicate is a grammatical function. In other words, a verb is a word you can look up in the dictionary whereas a predicate can only be found within a sentence. That’s because English grammar is primarily a description of the relationships between words.

Sidenote: If this all sounds confusing or needlessly pedantic, remember we make this kind of distinction in everyday life. One might say “1600 Pennsylvania Avenue,” and this refers to a specific place in the world. But a phrase like “my place” refers to a different address depending on the speaker. Only the U.S. President can say “my place” and also mean “1600 Pennsylvania Avenue.” And even then, his or her “place” is liable to change in four or eight years, when the new President is elected. In a similar fashion, each sentence has at least one “place” (i.e., one verb) that it calls home. And just like fancy people can have more than one “place,” fancy sentences can have more than one predicate.

So what exactly is a predicate? Practically speaking, it’s the main action or being verb in a clause. But this definition, while serviceable, is rather imprecise. (What’s a main verb as opposed to a not-main verb?) And in rare cases, this imprecision can lead to grammatical errors or unintended ambiguity. Readers should therefore have a passing familiarity with a more formal definition so they don’t fall prey to confusion down the line.

Formal Definition of Sentence Predicates

Formally speaking, a sentence predicate a word or phrase that determines what grammatical function all the other words in a sentence must play. I know how circular this reasoning sounds, but really it’s quite simple—if maddeningly counterintuitive—to understand and put into practice. Let’s look at a couple of example sentences:

I kick the karate master.

The karate master kicks me.

In this case, the preposition is kick(s), which is the main action described in the sentence. But again, don’t be fooled. It may be tempting to think the subject of the sentence determines the grammatical role played by the other words. After all, the subject comes first and changes the verb ending. (“I kick” becomes “master kicks.”) But this cannot be true because the verb “to kick” performs the same function in both sentences; which is to say, it names the action being performed. In contrast, the nouns swap functions depending upon whether they come before or after the preposition. In the first instance, “karate master” gets kicked; in the second, (a)he does the kicking. And what determines this difference? Precisely! Whether the noun comes before or after the preposition.

Now wait a minute, you might be saying. Can’t I just use the familiar word “verb” instead of this Latinate beast “preposition.” After all, it’s easy to understand that a verb is an action and a noun is a person, place, or thing. Can’t that be enough to determine the roles played by all the words in a sentence?

Unfortunately not. At least not in English. For one thing, it’s easy to show how a verb can form the grammatical subject (or object) of a preposition. Consider the following examples:

To kick is what I like to do.

Don’t make me kick, or you’ll get two!

Here the most “active” verb (to kick) is no longer the predicate, because it has been replaced in that role by two less “active” verbs. (I’m using scare quotes around “active” to avoid confusion with active and passive voice, which is an entirely separate though related question.) Sentence one uses a copula (“to be” verb) which quite easily takes a verb, adjectives, or any other part of speech as its subject. And sentence two uses the imperative mood (that is, a command) to push the “active” verb to the middle of the sentence, where it merely answers the question: what shouldn’t I make you do?

So here again, we have no choice but to work backwards from the predicate, do not, in order to understand the role of kicking in this sentence. But the same is not true of the second “main verb” in the sentence, which is “get.” That’s because get is also a predicate in its own right, and it could stand on its own as a complete sentence: You’ll get two.

Now we can see this sentence has two subject-predicate relationships, each forming its own grammatically complete clause.

Clause 1: Don’t make

Clause 2: You’ll get.

The meaning of everything else in the sentence is determined by its relationship to one or both of these clauses. And this is where the run-on sentence rule comes back into play. Because you should never just slam two such clauses together without using a word or phrase that explains how they are logically connected:

Don’t make, or you’ll get.

Otherwise, these two ideas are best left as separate sentences. Which is a subtle way to remind your reader the relationship is either nonexistent or unimportant.