Learn Finnish: Kirous murtukoon! - Let the curse break!

It is the 3rd-person singular imperative, also known as the jussive. It is formed by taking the infinitive stem of murtua (to break) and adding the suffix -koon. It expresses a command, strong wish, or concession directed at a third party, translating nicely to "let it break" or "may it break".

Why is the verb murtua used here instead of murtaa?

Murtua is the intransitive form of the verb, meaning the curse is breaking on its own. Murtaa is transitive, meaning to break something. If a wizard were breaking the curse, you would use murtaa, as in Velho murtakoon kirouksen! (Let the wizard break the curse!).

Could I change the word order to Murtukoon kirous!?

Yes, and it would sound very natural! In spells, dramatic declarations, or formal wishes, placing the jussive verb at the beginning of the sentence emphasizes the command and gives it a poetic tone.

How would I make this sentence negative ("Let the curse not break")?

You need the 3rd-person imperative negative verb älköön. The main verb then drops the ending and takes the suffix -ko or -kö. The negative sentence would be Älköön kirous murtuko!