Is 'burnt' acceptable as the past tense of 'burn'?
Burned, burnt: which one's right?
The answer is: yes.
If you're describing things—that is, using the past participle of burn as an adjective—you very well may find that burnt sounds better to your ear. Burnt sugar and burnt toast, for example, are both significantly more common in published, edited text than burned sugar or burned toast are. (Burnt also features in the color names burnt umber and burnt sienna.
But if you're using the past tense of burn as a ho-hum verb, talking perhaps about the toast you've just overtoasted, burned is likely to be your choice. Unless you're a speaker of British English or have been binge-watching "Sherlock." In American English, burned is usual past tense.
Usual or not, though, both burnt and burned are acceptable forms.
There was a time, by the way, when brent was a legitimate past tense too. That form seems to have peaked in the 1500s, but if you want to throw it into conversation just for fun we won't criticize.