spring / sprang / sprung.
So inflected. But "springed" is correct when the sense is "equipped with springs" {a springed mattress} {springed hinges} or "to spend the season of spring" {they springed in Europe}. (The latter usage will strike many readers as more than a little odd.)
The real challenge with these words is to get the past-tense and past-participial forms in their proper places. Some writers spring an erroneous "sprung" on their readers — e.g.:
o "They sprung [read 'sprang'] out of the gate with a string of initiatives that culminated with the president's 1993 budget victory." Clarence Page, "Clinton's Legacies Continue to Be a Work in Progress," Chicago Trib., 18 Jan. 1998, at 19.
o "Then they sprung [read 'sprang'] a backdoor play on Grimsley." Jeff Carlton, "Dudley Turns Corner, Makes Little Four Final," News & Record (Greensboro), 30 Dec. 1998, at C1.
The opposite misuse — "sprang" for "sprung" — is less common:
o "Some of these trade-terms may have originally sprang [read 'have originally sprung'] up as slang." Otto Jespersen, Mankind, Nation, and Individual from a Linguistic Point of View 147 (1946).
o "Bob Horne . . . said that there are more than 300 truckers' ministries that have sprang [read 'have sprung'] from the original, the Ministry of Transport, founded by Canadian chaplain Jim Keys in 1951." Sue Anne Pressley, "Truck-Stop Ministry Offers Comfort in the Odd Hours," Wash. Post, 3 Nov. 1996, at A3.
Language-Change Index — (1) "sprung" misused for simple-past "sprang": Stage 2; (2) "sprang" misused for past-participial "sprung": Stage 2.
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Quotation of the Day: "There are many people who won't listen until their ears are cut off." Georg Christoph Lichtenberg (1742-1799), in The Lichtenberg Reader: Selected Writings of Georg Christoph Lichtenberg 92 (trans. Franz H. Mautner & Henry Hatfield, 1959).