Learned - learnt

The past tense of '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, learnèd. (Some writers mark this pronunciation by writing the word with an accent on the second '-e-'.) For this reason, it may be preferable to use learnt for the usual spelling of the past tense.

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