English has a few verbs with two acceptable past forms, and burn is one of them. Both burned and burnt are standard, especially in writing. In many contexts, choosing one is a matter of style and region.
A useful rule of thumb is this: burned is often used as the regular past tense and past participle, especially in American English. Burnt is often used as an adjective, and it is more common in British English.
Here are some clear contrasts:
- Past tense (action): I burned the cookies because I forgot the timer.
- Past participle: The cookies were burned beyond saving.
- Adjective (describes a noun): I do not like burnt toast.
- Adjective in a fixed phrase: He has a burnt offering smell on his clothes.
That said, you will still see overlap. Some writers use burnt for the past tense, and it is not wrong in many varieties of English. If you want the safest choice for general audiences, use burned for the verb form and burnt when it is clearly acting like an adjective.