An old woman was burnt to death.
“was burned” indicates someone else’s doing, I think, i.e. passive. Otherwise it would just be “burned”.
It seems it’s not that easy.
For one, most of the other translations on tatoeba, that I can decipher, seem to agree with the finnish sentence, meaning effectively “An old woman died in a fire”. So I’m not sure whether my understanding of the english sentence is correct.
The second thing is, from what I read, that “burnt” is normally only the adjective, as in “The burnt cookies are still in the oven.”, while “burned” is used for the past tense, and probably the passive? Although not all sources agree, and some say that they are used interchangeably.