Writers are divided in opinion about punctuating introductory words or phrases—and often, they are at war with themselves with the topic. Even adherents of open punctuation will generally insert a comma after an adverb, whether it is transitional, like however, or descriptive, like suddenly, and will follow even a brief modifying phrase such as “according to the study” or “contrary to popular belief” with pausing punctuation.
But somewhere in between—in the case of a short opening phrase like “last year” or “in retrospect”—many people believe a comma setting the phrase off from the sentence’s main clause is unnecessary. For consistency, I advocate generally using a comma regardless of the phrase’s length, but even though I am a close-punctuation adherent, I realize there are exceptions.
Consider the use of please, for example. Read this sentence: “Please sit down.” Now, read this one. “Please, sit down.” Did you read them differently? I hope so. The intent behind each statement is distinct: “Please sit down” is an imperative barely tempered by a courtesy term; the person to whom the statement is delivered is expected to comply. By contrast, “Please, sit down” is an entreaty; the speaker sincerely hopes that the other person will accept the invitation.
There’s a difference, too, between “Of course you would say whatever you thought I wanted to hear” and “Of course, you would say whatever you thought I wanted to hear.” The first sentence is delivered with some heat; the speaker’s tone is wounded and derisive. The second statement, by contrast, is more measured and reflective.
These examples are more subtle than when one decides whether one should punctuate, for example, “In time you will understand why I acted as I did”: You either agree with me that if one is to punctuate a more extensive introductory phrase (“When you have time to reflect, you will understand why I acted as I did”)—and most writers will choose to do so—it’s only logical to treat a more concise opening phrase the same way (otherwise, where does one draw the line?), or you don’t agree. But sometimes, what a sentence communicates changes with the mere insertion or omission of a comma, and the writer should be sensitive to such nuances to help the reader read between the lines.
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Original post: A Comma (or Its Absence) Can Change a Sentence’s Message