Learned is the more common past tense and past participle of the verb learn. Learnt is a variant especially common outside North America. In British writing, for instance, it appears about once for every three instances of learned. In the U.S. and Canada, meanwhile, learnt appears only once for approximately every 500 instances of learned, and it’s generally considered colloquial.
Writers throughout the English-speaking world use learned as the adjective meaning possessing broad, profound knowledge. Incidentally, this sense of learned is pronounced with two syllables: LUR-ned. As a verb and in normal past-participial use, learned is one syllable.
Instances of learnt, as seen below, are especially easy to find in British publications:
What’s more, I learnt that it is possible for scientists to influence these enquiries. [Guardian]
But, just like Peter Siddle, he has learnt tricks from other sports. [Telegraph]
The same publications use learned much of the time, however.
But learned is the more common form, and it is used both in the past tense and as the past participle, as shown below:
He learned to read at a little schoolhouse where his parents had gone as well. [NY Times]
Although many new mothers think breastfeeding will be natural, it is a learned skill, she said. [News.com.au]
During his stay, he has learned that some things remain the same as on earth. [CBC.ca]
This Ngram visually renders the use of learned and learnt in a large number of U.S. books and periodicals published from 1800 to 2019.
And this one shows the words’ use in British books and periodicals from the same period:
Keep in mind, though, that both these Ngrams are skewed, even if only a little, by the adjectival learned.