Learn Finnish: Velho siunatkoon haltioita, joiden metsä palaa. - Let the wizard bless the elves, whose forest is burning.

Siunata is a type 4 verb. To form the third-person imperative (the jussive), you take the infinitive stem by dropping the final -a or -ä, which leaves siunat-, and then add the suffix -koon. This creates siunatkoon (let him/her/it bless).

Why is the object haltioita in the partitive case rather than the accusative/genitive?

The verb siunata (to bless) is a partitive verb, meaning it requires its object to always be in the partitive case. Because the wizard is blessing multiple elves, we use the partitive plural form haltioita.

Why is the relative pronoun joiden used here instead of jonka?

Joiden is the genitive plural form of the relative pronoun joka. It translates to "whose" here. We must use the plural form because it refers back to the plural noun haltioita (the elves). If the wizard were blessing only one elf, we would use the singular genitive jonka.

Could I use the verb polttaa instead of palaa to say the forest is burning?

No, because they behave differently. Palaa is an intransitive verb used when something is on fire, like the forest here. Polttaa is a transitive verb that requires an object, meaning someone or something is actively burning something else (for example, if the wizard were burning the forest).