wrong spelling for simple past of the verb- you meant to say "learned". "learnt" is the past participle and only used after "to have" for example "I have learnt"
I thought that both past simple and pp form of 'learn' have two different spelling that are -ed and -t.
Learnt is less common, but not incorrect. However, in America the preference is always for learned over learnt. https://en.oxforddictionaries.com/usage/learnt-vs-learned