Learn Finnish: Peliä pelattaessa kukin antaa yhden pelimerkin toiselle. - While the game is played, each gives one token to another.

Both words mean each or everyone, but kukin focuses more on individuals acting separately. In this lesson's context, it emphasizes that each individual person independently takes their turn to give a token.

Does the pronoun kukin always take a singular verb like antaa?

Yes. Even though kukin refers to multiple people in a larger group, grammatically it acts as a singular subject, just like each one in English. Therefore, the verb is in the third-person singular.

Why does the sentence start with pelattaessa instead of pelatessa?

Pelattaessa is the passive form of the temporal structure, meaning while being played or while people play. We use the passive here because the sentence does not specify exactly who is playing in that first clause. If it were a specific person, we would use the active form pelatessa, as in minun pelatessa (while I play).

Why is the game peliä in the partitive case, but the token yhden pelimerkin ends in -n?

This is a great reminder of the object rules. Playing a game is an ongoing action, so its object peliä is in the partitive. Giving a token is a completed action where the entire token is transferred, so its object yhden pelimerkin takes the accusative case, which looks exactly like the genitive with its -n ending.