When dependent clauses, prepositional phrases, and non-finite verb phrases function as modifiers, we need to know whether they are restrictive or non-restrictive. Restrictive modifiers are not punctuated; non-restrictive modifiers are, usually with commas. This calls for some explanation, of course.
A restrictive modifier is one that identifies exactly what the modified word refers to; in effect, a restrictive modifier gives essential information:
Disqualify any fleas who cheat in the dance contest. [Who cheat in the dance contest is a restrictive adjective clause that identifies the fleas who should be disqualified. No punctuation is possible.]
The tuffet at the end of the row once belonged to the famous Miss Muffet. [The prepositional phrase at the end of the row is a restrictive modifier of tuffet; it identifies the particular tuffet (a low footstool) that Miss Muffet was sitting on before the spider arrived—evidently, more than one tuffet is visible. Again, no punctuation is possible.]
The elephants who will be dancing the fandango in the competition need to get some sleep. [Who will be dancing the fandango identifies the particular elephants.]
In contrast, a non-restrictive modifier supplies additional information about something that has already been identified. A restrictive modifier couldn’t be left out without confusing the meaning; a non-restrictive modifier could be. Non-restrictive modifiers are enclosed in commas, showing where the modifier begins and ends:
Jack Horner, who seems to be doing something quite disgusting to his Christmas pie, once got a plum-pulling medal.
The person referred to is identified by his name, so the “who” clause is nonrestrictive and is set off from the rest of the sentence by commas. The “who” clause gives more ample information about Jack, but the essential meaning of the sentence is expressed by the bare structural backbone: Jack Horner once got a plum-pulling medal.
But suppose there were two Jack Horners in our circle (most likely through Facebook). We’d need some restrictive modification to identify which one we were speaking of:
The Jack Horner who hates plum pie will be lecturing on donuts and cupcakes today. [The adjective clause who hates plum pie is restrictive: it identifies the particular Jack Horner who will be lecturing. Note that the determiner (definite article) the is part of the identification.]
She’ll be comin’ ’round the mountain when she comes because her six white horses aren’t strong enough to get her over the top.
The finite verb, ‘ll be comin’, is modified by a prepositional phrase, ’round the mountain, and by two subordinating-connective clauses, the “when” clause and the “because” clause. All three are restrictive: ’round the mountain identifies the route of her coming; the “when” clause identifies the time of her coming (however indefinitely in the future); and the “because” clause identifies the reason for her comin’ ’round instead of comin’ over.
She’ll be comin’ ’round the mountain at about six-thirty, when rush hour lets up.
About six-thirty identifies the time of her comin’; the “when” clause gives additional information about the time. If we reversed the adverbs, the first would become restrictive and the second non-restrictive:
She’ll be comin’ ’round the mountain when rush hour lets up, at about six-thirty..
It may take a while to develop a feel for the difference between restrictive and nonrestrictive modifiers, and so in the following sections, I’ll be giving a number of examples of each kind of clause and phrase and will continue to comment on the distinctions between them.
WH-word Adjective Clauses
WH-word adjective clauses are commonly called relative clauses since they are felt to “relate” to the nouns they modify. (All modifiers relate to the phrases they modify, and so the term isn’t all that necessary, but you may run across it in other places, so I’m using it here.) In the first three examples below, the relative clauses are restrictive; in the second three they’re non-restrictive (I’ve italicized the relative clauses):
The duck who(m) you insulted is suing for damages.
The rules that were passed by the Migration Committee are for the birds.
The restaurant where you nibbled my finger has been declared a national eatery.
The Stringy-Cheeze Pizza Shoppe, where you first nibbled my finger, isn’t really a suitable site for a theme park.
Colonel Quack, who(m) you met last week, is too distracted to boycott the decoys.
The regulations about fooling around in formation, which were narrowly passed last week by the Migration Committee, are designed to get tough with clowns like Daffy.
In the first three examples, there’s no punctuation since the restrictive relative clauses identify what their modified nouns are referring to: the duck you insulted instead of, say, the duck you praised; the restaurant where you nibbled my finger instead of the one in which you nibbled celery; the rules passed by the Migration Committee instead of those passed by the Executive Committee. In the last three examples, the duck and the restaurant are identified by their names, and the regulation is identified by the prepositional phrase about kidding around in formation, so the italicized clauses in the last three examples are non-restrictive and are set off by commas.
If a non-restrictive element comes at the end of a main clause, only the first comma is needed. The second comma, so to speak, gives way to the punctuation of the main clause.
I adore my new CD of “The Quacking Sonata,” which I bought for chicken feed; it features Sidonia Mallard. [The non-restrictive relative clause ends with feed and is followed by the semicolon that separates the two independent clauses.]
He didn’t keep off of my blue suede shoes, which were pretty scuffed already. [The relative clause comes at the end of the sentence, where the period takes over.]
Non-finite Verb Phrases
On Friday the Pit Committee discussed the rules concerning off-duty hissing. [The italicized non-finite verb phrase identifies the rules, so it's restrictive and is not set off with commas.]
The Pit Committee’s new off-duty hissing guidelines, setting out more precise standards for choral harmony, were received with jubilant writhing. [New off-duty hissing identifies the guidelines, so setting out more precise standards for choral harmony is non-restrictive. It supplies more ample information about the guidelines and is enclosed by commas.]
I told Fang jokes to help him out of his depression, to revive his joie de vivre. [The two infinitive phrases, beginning with to help and to revive, both modify told. Either alone would be restrictive; it would identify the reason for the telling. But the second phrase is non-restrictive: it amplifies, by restating, the information supplied in the first phrase.]
The following three examples contain restrictive appositives:
My sister Sidonia is actually much more furry than I. [The apposed nouns are my sister and Sidonia. The lack of commas implies that I have more than one sister, and the name identifies the sister I’m talking about.]
My sister, Sidonia, is actually only somewhat more furry than I. [Now the commas tell us that the speaker has only one sister and is reminding us of her name.]
When I use the term “total idiot” to describe you, I’m being perfectly objective. [Total idiot identifies the term I use. Thus, it’s restrictive and isn’t enclosed in commas.]
Fang was amused by the fact that writhing breeches are no longer fashionable. [The noun clause that writhing breeches are no longer fashionable is in apposition with fact. It identifies the fact that amused Fang, so it’s restrictive and there’s no punctuation.]
The two following sentences contain non-restrictive appositives:
Sidonia, the well-known hieroglyphic dancer, will be performing during the migration. [The well-known hieroglyphic dancer supplies additional information about Sidonia, who has already been identified by her name.]
Nowadays, I’m fully occupied with my most recent project, eating chocolate bunnies.
Adverb Phrases and Clauses Following a Verb
Call me as soon as Fang to starts act up. [The "as soon as" clause identifies the time when I should be called.]
We stuffed him in his basket. [The prepositional phrase identifies the place where he was stuffed.]
They love him because he’s a pachyderm. [The adverb clause identifies the reason he's loved.]
Let him go if he hollers. [The adverb clause identifies the conditions under which he is to be released.]
His motives remained shrouded in murky mystery. [The non-finite verb phrase beginning with shrouded identifies the manner of remaining.]
The italicized phrases and clauses in the following two examples supply information that amplifies or comments on what the verb states. Thus, they are nonrestrictive and are set off with commas:
The scouts are joining the circus, where they intend to practice for their lion-tickling badges. [The place where the scouts are going is identified in the main clause, so the adverb clause gives additional information. If the comma is left out, what is implied is that there’s more than one circus in the picture, and the writer needs to distinguish between them.]
I love her for her bravery, for the courageous way she teases Fang. [The second prepositional phrase amplifies the idea of bravery.]
The snakes were already coiled when we arrived, although we had expected to use our new cobra coilers. [The “when” clause, which modifies were coiled, is restrictive: it identifies the time when the coiling had already been accomplished. The “although” clause doesn’t identify any circumstances connected with were coiled (where, when, why, or how, for instance) but gives additional information—it concedes that we had had expectations.]
If an adverb clause or phrase that modifies the main verb comes at the beginning of the main clause, it’s usually followed by a comma even if the clause or phrase would be restrictive if it followed the finite verb:
Until Hell freezes over, I’ll hang on to my ice-cube trays.
Although she acts truculent, Miss Hiss can be succulent.
To stuff an elephant, you need first to find out what he likes to eat.
Following the séance, Miss Hiss seemed thoughtful.
Obviously, pink frogs have a slight camouflage problem.
It’s possible to leave the comma out if the modifying clause or phrase is short and if there’s no question of where it ends. For example, we could leave out the commas in the first four examples above. However, if there’s any chance at all of misreading—of not seeing where the introductory clause or phrase ends and the following one begins—the comma is necessary. In the last example, the comma makes it clear that obviously modifies have, but without the comma it would apparently modify pink. Here are some other potentially ambiguous situations:
After all, pleading with a flea is humiliating. [Without the comma we might (temporarily) read after all pleading as a prepositional phrase.]
Actually, nasty bunnies aren’t all that dangerous. [Without the comma, it might look as though actually modifies nasty instead of aren’t.]
When one is migrating, great distances seem piddling. [Without the comma, great distances might appear to be the completer of is migrating.]
Although they look yucky, toads have a lovely texture. [The comma prevents us from reading yucky toads.]