Learn Finnish: Onko tämä kala karhuille vai ketuille? - Is this fish for the bears or for the foxes?

We start with the dictionary word karhu (bear). To make it plural in this case, we add the plural marker -i-. Then we add the allative ending -lle, which here translates to "for". Because karhu ends in the vowel -u, the -i- simply attaches right next to it without dropping any letters, giving us karhuille.

Why does ketuille only have one 't' when the dictionary word for fox is kettu?

This is due to consonant gradation, a rule you learned in earlier lessons. Adding the plural -i- and the ending -lle closes the syllable (ke-tuil-le). This "closed" syllable forces the strong double tt in kettu to weaken into a single t, giving us the stem ketu- before the endings are added.

Does the -lle ending always mean "for"?

Its most literal meaning is "onto" (like putting something onto a table). However, Finnish uses this exact same case to show who receives an action or who something is intended for, which is why it translates nicely to "to" or "for" in sentences like this.

Why do we use vai here instead of tai for the word "or"?

In Finnish, vai is strictly used in questions when you are giving the listener mutually exclusive options and they must pick one (it is either for the bears OR the foxes). You use tai in normal statements, or in questions where the options aren't mutually exclusive.