In my six semesters as an English major, this is the best thing I learned: When in doubt, put the best bit of a sentence at the end, the next-best bit at the beginning and the rest in the middle. So in order of bestness, that's 2, then 3, then 1.
Photo by Brad Neathery
What's the "best bit"? It might be the bit that sounds prettiest. It might be the bit that gets at your larger point. It might be the most specific or surprising word.
It might only be the "best bit" in the context of the rest of the sentence. In his book How to Write a Sentence, Stanley Fish praises this opening line, from a student's essay:
I was already on the second floor when I heard about the box.
On its own, "the box" is a mundane phrase, but that's what gives it so much power here. By the time you read "when I heard about," you're more ready for an ending like "Mother's death" or "the explosion." No, it's a box! The box! What's in that box!?
Most sentences you write, you won't bother parsing out like this. The 2-3-1 structure does its best work in opening lines. Like the opening line of One Hundred Years of Solitude:
Many years later, as he faced the firing squad, Colonel Aureliano Buendía was to remember that distant afternoon when his father took him to discover ice.
Or the opening line of Beloved:
124 was spiteful.
Every part of the 2-3-1 rule is flexible, because it's not a rule. You can put the best bit at the beginning. You can make a whole sentence out of the best bit. And if a sentence is already working, don't break it just to match the pattern. The opening line of The Voyage of the Dawn Treader goes 3-2-1:
There was a boy called Eustace Clarence Scrubb, and he almost deserved it.
Sometimes breaking the rule is the point. The opening line of The Bell Jar is an arch 2-1-3:
It was a queer, sultry summer, the summer they electrocuted the Rosenbergs, and I didn't know what I was doing in New York.
Of course, by now you've disagreed with at least one of these examples, because this is all subjective and finicky and a little fake. The least-interesting part of a sentence might be doing the most work to introduce a story or essay, or it might be establishing a tone. You might find a good reason to structure your sentence differently.
That's fine too! It's just useful -- if I can add one last mealy-mouthed caveat -- to think carefully about how you order your thoughts at the sentence level.
But sometimes you should take the 2-3-1 rule literally. Try it whenever you pause while putting a sentence together, or whenever you wonder why your brilliant thoughts are falling flat. Then write another sentence, and try it again.