When do you use “learnt” and when “learned”?

Solution 1:

Merriam-Webster marks learnt as "chiefly British", and Wiktionary as "UK", adding that learned is the "Standard US English spelling". Quoting a linguist's comment from elsewhere,

[The Corpus of Historical American English] shows that learned has always been more common than learnt in American English. At least, since 1810.

So it's not like learnt is completely unheard of in Americal English, but learned has always been more popular, and according to the COHA timetable, the usage of learnt has been on a more or less steady decline since 1820:

_{(X axis: year, Y axis: incidences per million words.)}

Nowadays, according to the Corpus of Contemporary American English, learnt is most popular in the context of fiction and academic publications, and least popular in newspapers:

           learned   learnt     RATIO

SPOKEN        9370       18     520.6
FICTION       9624       88     109.4
MAGAZINE     11924       18     662.4
NEWSPAPER     9224        6    1537.3
ACADEMIC      8921       96      92.9

What's more, even the British National Corpus has more cites for learned than for learnt. The stats look as follows:

           learned   learnt     RATIO

SPOKEN         161      291       0.6
FICTION       1066      409       2.6
MAGAZINE       341      171       2.0
NEWSPAPER      459      151       3.0
NON-ACADEMIC   778      321       2.4
ACADEMIC       837      273       1.6
MISC          1588      537       3.0

It is worth noting that Merriam-Webster, Wiktionary, the American Heritage Dictionary of the English Language, and Collins English Dictionary all list learned as both a verb form and an adjective, but learnt only as a verb form.

Lastly, here's a related question: Dreamed vs. Dreamt, Leaped vs. Leapt, Lighted vs. Lit.

Solution 2:

In American English, "learned" is the usual spelling; "learnt" is too rare.

In British English, both "learned" and "learnt" exist for the verb, but the adjective (as in "the learned professor") is always "learned". (Sometimes, especially archaically, "learnèd".)

Solution 3:

[Learniv]

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.”

But which is the most used one? Let´s look at Google Ngram Viewer tool which displays a graph showing how phrases have occurred in a corpus of books over the years.

Learned is more used, but bear in your mind that „learned“ is also an adjective, so comparing with a verb is not representative.

Source: [Learniv]