burned / burnt
The two past forms of burn.
Burned is the usual American form for the past and the past participle: The toast burned. The toast was burned. Burnt is more common in British English and survives in American English in set phrases and as an adjective: burnt toast, burnt offering. After forms of have and forms of be, prefer burned unless the set phrase is fixed.
Examples
Use She burned the letter.
Use The letter was burned.
Use burnt toast
Avoid She has burnt the letter. (As the default American form.)