Commas in nonessential expressions
In many sentences, you can tell whether an expression is nonessential or essential by trying to omit the expression. If you can leave it out without affecting the meaning or the structural completeness of the sentence, the expression is nonessential and should be set off by commas.
Nonessential: We will consult with Dr. Smith, who has expertise in neurological disorders in neonates.
Essential: Let's get a consult with someone who has expertise in neurological disorders in neonates.
Nonessential: There is, no doubt, a reasonable explanation for his elevated LFTs.
Essential: There is no doubt his elevated LFTs can be reasonably explained.
In other sentences, the only way you can tell whether an expression is essential or nonessential is by the way it is said aloud. If the dictator's voice drops when the expression is uttered, it is most likely nonessential; if the voice tends to rise, the expression is most likely essential.
Nonessential: The cardiologist and the oncologist agreed, therefore, that treatment of the cancer would have to be delayed pending treatment of the patient's cardiac condition.
Essential: The cardiologist and the oncologist therefore agreed that treatment of the cancer would have to be delayed pending treatment of the patient's cardiac condition.
Note: The word that most often introduces essential clauses and the word which most often introduces nonessential clauses.
Setting off a nonessential word, phrase or clause implies that it can be removed from the sentence without compromising the meaning or structural integrity of the sentence. When in doubt about whether a clause is essential or nonessential, construct the sentence without the questionable portion; if its integrity and structure remain intact, it is a nonessential clause.
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