When would one use "burnt" and when would "burned" be more appropriate?

Solution 1:

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.

(For complete table use the compare tool between burned and burnt in the COCA.)

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").

Solution 2:

Burnt toast is easier to say than 'burned toast'. And some things said simply become the more commonly accepted way to say it and I think that's the case with 'burnt toast'.

There is also a shadow of some mountain regions of the South, of dialects of British English, of 'folk speech' in the word 'burnt' and others like it. My father in everyday discourse would say burned, dreamed, spelled but when with his brothers and mountain-raised Scot-Irish father would only say 'burnt, dreamt, and spelt as in 'Remember when Jim took the prize because he spelt so well?"

Solution 3:

The simple past and past participle of burn is either burned or burnt.

In addition, burnt also functions as an adjective to describe something as being damaged, injured, or made black from burning, a meaning slightly different from it being used as a participial adjective.