Q: Many people use “when” and “once” interchangeably, as in “We can focus on polishing the text once the content is closer to being final.” I know they sort of sound alike, but is it correct to use “once” when you mean “when”?
A: The short answer is that the two words overlap somewhat and both can be used as conjunctions to mean “as soon as” or “after,” though “once” seems a bit more emphatic than “when” here. Now for the longer version.
The word “once” has worn many hats since it showed up in Anglo-Saxon times. It’s been an adjective, an adverb, a noun, and a conjunction.
When “once” first appeared in Old English more than a thousand years ago, it was an adverbial form of the noun “one,” and meant “at one time only.”
The earliest example in the Oxford English Dictionary (with “once” spelled “ænes”) is from the Lambeth Psalter (circa 1000), a manuscript with Latin and Old English text from the Book of Psalms:
“Semel iuraui in sancto meo : ænes ic swor on minum halgan” (“once have I sworn in my holiness”).
The spelling evolved gradually from “ænes” to “ones” to “once.” As John Ayto’s Dictionary of Word Origins explains, the modern “c” spelling reflected “the fact that that once retained a voiceless s at its end, whereas in ones it had been voiced to z.”
The adjective “once” showed up in the mid-1500s, but it wasn’t until the early 1600s that it took on its modern sense of “former.”
The first example in the OED is from Swetnam the Woman-Hater (1620), an anonymous comedy about a misogynist tried by a court of women:
“Magnanimous Ladie, maruell not, / That your once Aduersary do’s submit himselfe / To your vnconquer’d beautie.”
The noun “once” showed up in writing around the same time. The first Oxford example is from A Newyears Gifte, a 1579 poetry collection by Bernard Garter: “Once is no custome.”
The conjunction “once,” the usage you’re asking about, showed up before both the adjective and the noun. The earliest citation in the OED is from Ordinal of Alchemy (circa 1477) by Thomas Norton: “Metalle ons metalle shal not more encrese.”
And here’s an example from Ludus Literarius, a 1612 book by John Brinsley about education: “Once gotten, they were easily kept by oft repetition.”
Finally, this example is from Samuel Richardson’s novel Clarissa (1747): “No peremptoriness, Clary Harlowe! Once you declare yourself inflexible, I have done.”
We won’t get into the etymology of “when” now, except to note that it’s ultimately derived from an ancient interrogative root reconstructed as qwo-, according to Ayto’s Dictionary of Word Origins.
Ayto adds that the ancient root has also given English the word “quandary,” which is the source of many of the questions that we answer on our blog.