Learn Finnish: Järjestäjä unohti pukukoodin, joten juhla oli rento. - The organizer forgot the dress code, so the party was relaxed.

Because the action is complete. The organizer completely forgot the specific dress code, making it a total object. In Finnish, total singular objects take the accusative case, which looks identical to the genitive with its -n ending. If the sentence were negative (did not forget), the object would take the partitive.

What is the difference between the verb unohtaa used here and the similar verb unohtua?

Unohtaa is an active, transitive verb meaning 'to forget', which requires a subject doing the forgetting and an object being forgotten. Unohtua is an intransitive verb meaning 'to be forgotten' or 'to slip one's mind'. If we used unohtua, the sentence would be formulated differently, such as Pukukoodi unohtui järjestäjältä.

How does the conjunction joten differ from siksi if they both mean 'so' or 'therefore'?

Joten is a conjunction used to link two clauses where the second is a direct consequence of the first, acting just like the English 'so'. Siksi is an adverb meaning 'for that reason' and usually starts a new independent sentence, or it requires a conjunction like ja before it, such as in ...ja siksi juhla oli rento.

Could we use the plural juhlat instead of the singular juhla here?