Learn Finnish: Kukin kierros on jännittävä, vaikka pakka on sama. - Each round is exciting, even though the deck is the same.

Kukin modifies the subject of the sentence, kierros (round). Because the subject is in the basic nominative case, the pronoun must match it. If the sentence was about 'the rules of each round', you would need the genitive form to match: kunkin kierroksen säännöt.

Could I use jokainen instead of kukin to mean 'each round'?

Yes, jokainen kierros is perfectly grammatical and means roughly the same thing. However, kukin has a more distributive feel. It emphasizes looking at every individual round separately, which perfectly highlights the contrast that the deck stays the same but every single round is a unique experience.

Why is the adjective jännittävä (exciting) in the nominative case instead of the partitive?

When describing a subject with the verb olla (to be), the adjective's case depends on whether the subject is countable or uncountable. Because a round (kierros) is a distinct, countable thing, the adjective takes the basic nominative case. If the subject were an abstract or uncountable concept like playing, you would use the partitive jännittävää.

Does the word pakka always refer to a deck of playing cards?