Learn Finnish: Kuka osaa uida hyvin?

The basic dictionary form is osata (to know how to / to be able to). It is a Type 4 verb. To conjugate it, drop the -ta to get the stem osa-. Because the subject is a third-person singular word, we double the last vowel of the stem, giving us osaa.

Why is uida in its basic dictionary form instead of being conjugated?

This is a verb chain! When you use a verb like osata together with another verb, only the first verb is conjugated to match the subject. The second verb, uida (to swim), stays in its basic infinitive form.

Does the question word kuka always take the "hän" (he/she) form of the verb?

Yes. When kuka (who) is the subject of the sentence, it is treated as a singular third person. That is why we use osaa (the same form used for hän) rather than any other form.

I recognize the word hyvä (good). Is hyvin related to it?

Exactly! Hyvä is the adjective "good", while hyvin is the adverb "well". Because we are describing how someone swims (an action), we use the adverb hyvin, just like in English.