Learn Finnish: Olit harjoitellut paljon, kyllä sinä osaat. - You had practiced a lot, you certainly know how.

Here, kyllä acts as an epistemic adverb—a word that indicates the speaker's degree of certainty. Instead of meaning "yes", it asserts the absolute truth of the statement, acting like "certainly", "surely", or "I have no doubt". It adds a strong tone of reassurance to the sentence.

Why does the sentence jump from the past perfect (olit harjoitellut) to the present tense (osaat)?

The tenses map out the timeline of cause and effect. Olit harjoitellut (you had practiced) describes an action completed deep in the past, before the current situation. Osaat (you know how) is in the present tense because the result of all that past practice is an active, ongoing skill you possess right now.

Could I use voit instead of osaat to say "you can" here?

No, because Finnish separates different meanings of "can". Osata specifically means to know how to do something through learned skill—which perfectly matches the fact that you practiced (harjoitellut). Voida refers to physical capability or opportunity, and saada means having permission.

Is the pronoun sinä mandatory here, or could I just say kyllä osaat?