Let’s imagine a sentence is a grownup with a job. It doesn’t live at work, though. Nope. It functions equally well away from its paragraph workplace and out on its own because a sentence is fully independent, thankyouverymuch. And independence is not the focus of this post. (Need to review sentence types and the proper punctuation for them? Go here.)
If sentences are grownups, dependent clauses are teenagers still living at home. Occasionally one of them may get mistaken for a sentence because clauses, like sentences, contain a subject and a verb. However, dependent clauses don’t work independently like sentences do because they also include words that force them into dependency (more on that later). Think of them as belonging to a sports team. They have an important role to fulfill, but the position each one plays is just one cooperative component of the team’s action. The actions of a goalie, or a running back, or a shortstop don’t make much sense to an audience if played alone.
Like teenagers, dependent clauses get categorized by what they do around others. (And this is where my comparison stops, for goodness sake. It would be stretching it too far to nickname clauses after junior high school cliques. But then again, there is something very intriguing about some of the sentences in an argument essay containing goth clauses, isn’t there? )
When a clause is not essential to the meaning of the sentence then that clause is categorized as a nonrestrictive clause. (Nonrestrictive is also know as nonessential or non-defining depending on which of the terms was in fashion when your junior-high English teacher taught it). Whatever label you learned for it, the important thing to remember is that nonessential information is separated from the rest of the sentence with commas.
If sentences are grownups, dependent clauses are teenagers still living at home.
In the example above, the dependent clause (shown in blue) adds comparison information to the metaphor. However, it is not essential to mention sentences in order to make a metaphor involving dependent clauses and teenagers. Since the clause is nonrestrictive, a comma separates it from the rest of the sentence.
Now look at this example:
Occasionally one of them may get mistaken for a sentence because clauses, like sentences, contain a subject and a verb.
The words in orange are extra but nonessential information. They interrupt the main sentence, so they are set apart by commas. However, they do not contain both a subject and a verb, which is a defining requirement for clauses. “Like sentences” is only a phrase.
If, however, the information is part of the meaning of the sentence, the clause is labeled restrictive (or essential or defining, see caveat above). This clause does not get commas.
If the clause is not essential to the meaning of the sentence that clause is labeled nonrestrictive.
In the above example, the clause (shown in blue) is required: it is essential information because it defines “that clause.”