Oh, my friends, why must this question even be raised?
Of the grammar and usage superstitions that I have students look into (and on which I poll them each semester), this is one that they have consistently been taught by teachers at lower levels. It’s definitely in the top five superstitions, along with “don’t use contractions,” and never begin a sentence with ‘and,’ ‘but,’ or ‘because’). That qualifies it as pernicious in my book.
Some superstitions have traceable origins (especially the ones that try to impose Latin syntax). This one seems to have no clear source. My working hypothesis is that some instructors teach structure too zealously: when they drill students in both the 5-paragraph essay format and the topic sentence-supporting sentences-concluding sentence paragraph structure they send this message. They might not actually say “never write a one-sentence paragraph!” (although I suspect they do), but this is the message that many of their students take away.
The one-sentence paragraph is, of course, perfectly acceptable in English. As it also is, I assume, in every other written language. Why wouldn’t it be?
Paragraphs and paragraph breaks exist for a number of reasons, ranging from the obvious (logical organization of ideas) to the more subtle (psychological breaks for the reader otherwise facing a seamless wall of text). Sometimes a paragraph might need many sentences to accomplish its goal; sometimes only one.
It’s difficult to pick up a book of any kind and not find a one-sentence paragraph in short order. One source has noted that between 15% and 20% of all paragraphs in English writing are only one sentence long. (…I’m kicking myself, because I haven’t been able to locate this reference: I thought it was in Chicago 16, but I can’t find it tonight. If you know where to look, help me out.) That figure seems about right, because even in many technical texts it’s difficult to read more than a page or two without coming across one.
Consider this example, also:
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
That’s the Preamble to the United States Constitution, in case you didn’t recognize it immediately. It’s a single sentence, making a single paragraph. If it was good enough for Jefferson and Adams, it’s good enough for me. It also happens to be one of my favorite paragraphs, regardless of sentence count.
What are the most common uses of the one-sentence paragraph? Dialogue is one, of course:
“Is it acceptable to use a one-sentence paragraph?” he asked.
“Sometimes you’d be a fool not to,” she replied.
You also might need to use one-sentence paragraphs when quoting a source. It’s entirely conceivable that the paragraph you’ve quoted contained only one sentence. If so, depending on how it’s been integrated into your material, it might stand alone in your text as well.
One-sentence paragraphs are frequently used for emphasis. After a longer passage, for example, an author might hammer a point home or summarize with a short, single-sentence paragraph. Fiction and non-fiction both can make great use of this technique. Usage guides and other advice-givers often stress that this tactic shouldn’t be abused. Some of them get too strident in their admonition, but in general they’re right: if too many of your paragraphs (discounting dialogue) are only one-sentence long, the reading can get tedious. Using too many of them weakens their effect. Like so many of the tools in a writer’s kit, it needs to be used appropriately.
Journalists (or their editors) have a bad habit of over using one-sentence paragraphs. A student of mine once provided a newspaper article as an example for the class of one-sentence paragraph use “in the wild.” It was a short article, perhaps 15 sentences long. And each sentence was its own paragraph. This would be extreme in any other form of writing, but usage guides often shrug at this, adopting a fall-back position that says, in effect, ‘where journalism is concerned the rules of good paragraph style no longer apply.’ It’s more appropriate to say that some journalists, just like some writers, don’t use good style to begin with.
When it comes to one-sentence paragraphs, give yourself permission to use them. Just don’t abuse them.