This verb has both an irregular and regular form. You can use both and both are correct. Speakers in North America and Canada use learned while the rest of the English-speaking world seems to prefer learnt.
Learned (but not learnt) is also an adjective. When said of a person, it means ‘ having a lot of knowledge because you have studied and read a lot`.
It can also be used of objects, meaning ‘showing, requiring, or characterized by learning’.
Example: She is a ‘very learned’ woman.”