What is the difference between learned and learned?

The British English way of the past tense Word learn is learnt. The American English way of the past tense Word learn is learned.

"Learnt" is more used in UK and Canada while "Learned" is more used in USA

In addition, either term can be used as an adjective ("the learnt material" vs "the learned material") following the same rules.

Also, there is a 2 syllable Word, learnéd (though the accent on the e has been more or less completely dropped since the advent of modern English, meaning you'll only see the accent in Shakespeare's works and that of his contemporaries or predecessors). "Learnéd" (pronounced learn-ed) is an adjective meaning that the subject has done a lot of learning. It is more or less synonymous with "wise."