Which is Correct "Learned" or "Learnt" ?

As is often the case, this is one of the differences between American English and British English. They are, of course, both forms of the past tense and past participle of the verb 'learn', which is an irregular verb.

Americans tend to say learned and the British tend learnt. Although both can be considered correct.

Other verbs that have this characteristic are: burn, dream, kneel, lean, leap, spell, spill, spoil.

All of these are also irregular verbs.