Commas, Conjunctions, and Modifiers

Many writers and even editors have trouble deciding where to place the commas in a sentence in which a modifying phrase or clause appears after a conjunction. The following are correctly punctuated examples of such sentences:

He scratched his head and, fretting over the punctuation, added a comma.

She forgot to add the commas, and if it were a test she would have failed.

She omitted the commas, and, to her regret, no one understood her sentence.

When punctuating these types of sentences, you must first determine whether the conjunction joins two independent clauses or verbs in a compound predicate. (In a compound predicate, two verbs share a subject.)

Let’s look at the first example.

Compound Predicates

He scratched his head and, fretting over the punctuation, added a comma.

Here, the conjunction (and) joins verbs (scratched and added) in a compound predicate.

Placing commas in this type of sentence is straightforward:

• Do not place a comma before the conjunction in a compound predicate.
• Always use commas around phrases (such as fretting over the punctuation) that intervene in a compound predicate.

Now let’s look at the next two examples.

Independent Clauses

She forgot to add the commas, and if it were a test she would have failed.

She omitted the commas, and, to her regret, no one understood her sentence.

In each of these examples, the conjunction joins two independent clauses and thus a comma appears before and. A comma generally precedes a conjunction that joins two independent clauses. (The comma is optional if the clauses are short and closely related: “Lightning appeared and thunder soon followed.”)

But why are there commas around to her regret but not around if it were a test?

Let’s look at each example separately.

Independent Clauses with Essential Modifying Phrase or Clause

She forgot to add the commas, and if it were a test she would have failed.

The Principle. A short modifying phrase or clause that is essential to the meaning of the sentence should not be set off from the rest of the sentence by commas at both ends. Thus, in the example above, no commas are used around the modifying clause if it were a test since the clause is short and essential to the meaning of the sentence.

The Test. You can test whether the phrase or clause is essential by omitting it. If the resulting sentence makes no sense, the phrase or clause is essential. In the example above, the sentence cannot read, “She forgot to add the commas, and she would have failed.”

Other Considerations. If the essential modifying phrase or clause is more than a few words long, however, you should place a comma at the end of it, to make the sentence easier to read:

She forgot to add the commas, and if it were a test instead of a homework assignment, she would have failed.

Though not necessary, you could also insert a comma for readability after the short clause if it were a test:

She forgot to add the commas, and if it were a test, she would have failed.

Independent Clauses with Nonessential Modifying Phrase or Clause

She omitted the commas, and, to her regret, no one understood her sentence.

The Principle. A nonessential phrase or clause must be set off from the rest of the sentence. In this sentence, to her regret is not essential to the meaning of the sentence.

The Test. The sentence could read, “She omitted the commas, and no one understood her sentence.” Because the phrase to her regret can be omitted, it must be set off from the sentence.

Other Considerations. Some writers may prefer to omit the comma before and to avoid hemming in the conjunction. The comma before and is optional because the independent clauses are short and the comma is not needed for clarity:

She omitted the commas and, to her regret, no one understood her sentence.

You could also, as Claire Kehrwald Cook suggests, “substitute a pair of dashes for the commas enclosing the modifier,” or you could “transpose the modifier” (128):

She omitted the commas, and—to her regret—no one understood her sentence.

She omitted the commas, and no one understood her sentence, to her regret.

Cook also notes that when a transitional adverb, such as fortunately, blends into a sentence, you could use commas around the term if you wish to emphasize the adverb, but they aren’t necessary (125):

She omitted the commas, but fortunately everyone understood her sentence.

But if the adverb separates the verb from its object, then commas are needed (126):

She remembered, fortunately, to add the commas, and everyone understood her sentence.

Work Cited

Cook, Claire Kehrwald. Line by Line: How to Edit Your Own Writing. Houghton Mifflin, 1985.