Learned - learnt

The past tense of the irregular verb 'to learn' can, like that of 'to spell', be spelled in either of two ways - learnt and learned. However, as an adjective to mean 'scholarly', 'erudite', it must be learned - which is then pronounced with two syllables, 'LEAR-ned', IPA: /ˈlɜː^rn ɪ (or ə)d/. (Some writers mark this pronunciation by writing the word with an accent on the second '-e-' as learnèd.) For this reason, it may be preferable to use learnt for the usual spelling of the past tense.

AWE also has a page on the irregular verb 'to learn'.

You may also want to check the distinction between learn and teach.