Learn (to gain knowledge): learn > learned > learned
Learned is the standard past-tense verb in both American and British English. Although the variant learnt appears more frequently in British English, it is still so uncommon that it can safely be considered an affectation best avoided. Don’t be distracted by Bryan A. Garner’s vague description that, “in BrE, it’s often learnt,” which implies that British English writers prefer that spelling—they do not.
Pronounce learned with one syllable when it’s a verb and two syllables when it’s an adjective.