Is learnt UK English and learned US? Is it that simple?
I’m used to using learnt, but my US spellchecker says it is wrong.
[The Corpus of Historical American English] shows that learned has always been more common than learnt in American English. At least, since 1810.
So it's not like learnt is completely unheard of in Americal English, but learned has always been more popular, and according to the COHA timetable, the usage of learnt has been on a more or less steady decline since 1820:
(X axis: year, Y axis: incidences per million words.)
Nowadays, according to the Corpus of Contemporary American English, learnt is most popular in the context of fiction and academic publications, and least popular in newspapers:
learned learnt RATIO SPOKEN 9370 18 520.6 FICTION 9624 88 109.4 MAGAZINE 11924 18 662.4 NEWSPAPER 9224 6 1537.3 ACADEMIC 8921 96 92.9
What's more, even the British National Corpus has more cites for learned than for learnt. The stats look as follows:
learned learnt RATIO SPOKEN 161 291 0.6 FICTION 1066 409 2.6 MAGAZINE 341 171 2.0 NEWSPAPER 459 151 3.0 NON-ACADEMIC 778 321 2.4 ACADEMIC 837 273 1.6 MISC 1588 537 3.0
It is worth noting that Merriam-Webster, Wiktionary, the American Heritage Dictionary of the English Language, and Collins English Dictionary all list learned as both a verb form and an adjective, but learnt only as a verb form.
Lastly, here's a related question: Dreamed vs. Dreamt, Leaped vs. Leapt, Lighted vs. Lit.
In American English, "learned" is the usual spelling; "learnt" is too rare.
In British English, both "learned" and "learnt" exist for the verb, but the adjective (as in "the learned professor") is always "learned". (Sometimes, especially archaically, "learnèd".)