Which is correct, “learnt” or “learned”?

Answer 1

They’re both correct. “Learnt” and “learned” are real, actual words that have legitimate uses in modern English. However, these two words aren’t fully interchangeable, and there are differences between them. Also, “learned” has more than one meaning.

In every English-speaking country except for the United States and Canada, the word “learnt” is the past tense, as well as the past participle, of the verb “to learn”. For example:

1. Present: Christine and her friends learnat school each day.
2. Past: Last year, my cousin learnt to speak Portuguese.
3. Present: I typically learn faster when the material interests me.
4. Past: We learnt how to drive when we were teenagers.

Remember, though, that it does matter which side of the pond you’re on. In both American and Canadian English (or, more efficiently, North American English), the word “learnt” is very seldom, if ever, used. It practically doesn’t exist. I’m American and in retrospect I can’t think of a single time—ever—that I’ve used the word “learnt” in any context. And I’m a writer, at that.

In other words, in the two sentences marked “past” in the bulleted examples above, the word “learned” replaces “learnt” in the United States and Canada:

1. Last year, my cousin learned to speak Portuguese.
2. We learned how to drive when we were teenagers.

“Learned” can also be an adjective, in which case it is pronounced as two syllables. This meaning, this use, of “learned” is the same throughout the English-speaking world. As an adjective, and especially when said of a person, “learned” means “having much knowledge acquired by study”. A few examples:

1. Dr. Green was a wise, learned [learn•ed] woman who exuded dignity wherever she went.
2. They decided on the owl logo design, agreeing that the owl symbolizes being learned [learn•ed] and scholarly.

Answer 2

Both are correct in terms of "learnt" being the British English spelling and pronunciation of the American English equivalent "learned". They both mean the same thing and are used similarly grammatically too.

Answer 3

Technically, both words are used as the past participle and past tense of the verb "to learn", so you could use either. However, in the United States, learned is the correct version that should be utilized.

References:

https://www.dictionary.com/browse/learned?s=t

https://www.dictionary.com/browse/learnt?s=t

https://www.ecenglish.com/en/social/blog/ec-central/2016/10/10/learnt-or-learned#:~:text=Both%20versions%20of%20the%20word,is%20favoured%20in%20British%20English.