To quote Webster's Comprehensive Dictionary (1988, Encyclopaedic Edition, Volume 1, p. 726): "learn (lurn) v. learned or learnt..." On your logic, examples such as spelt, dreamt, and knelt should only be spelled, dreamed, and kneeled - Both are correct, of course. Furthermore, Ask Oxford dot Com puts the difference down to the original British vs the American adapation: "-nt" is generally used in British English, whilst "-ned" is the Americanism. I <3 Leroy