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.
North can also be used as a noun: one of the points of the compass is called north, and "the north" is frequently used to refer to geographic regions. Northern is always an adjective (notwithstanding its occasional use as a proper noun, as in "I attended college at Northern").
If you had to choose one of them to use as an adjective, you would probably want to use northern. North as an adjective is generally limited to proper nouns or idiomatic/traditional usage: the North Pole, the North Star, a north wind, the north face of the mountain, the north end of town.
Best Answer
They are both used as the past tense of burn.
In American English, burned is used much more frequently than burnt. In the Corpus of Contemporary American English, a search for burned returns more than 5 times more results than a search for burnt (11558 for burned vs. 2005 for burnt). Note that in combination with certain nouns, burnt is actually what is in general used: "burnt ivory", "burnt cobalt", "burnt orange", "burnt yellow", "burn toast", "burnt smell", etc.
In British English, hovewer, they seem to be used with more similar frequency. In the British National Corpus, a search for burned returns 1435 results and a search for burnt returns 1252 results. Unlike American English, there are many occurrences of burnt as a verb too (e.g. "And I burnt them").