Learn Finnish: Jos nukkuu tarpeeksi, oppii paremmin. - If one sleeps enough, one learns better.

There is no specific subject! This is the zero-person construction. By using a 3rd person singular verb without a subject pronoun like hän, Finnish creates a general statement meaning 'if one sleeps' or 'if you sleep in general'.

Does a zero-person sentence always leave out the subject pronoun?

Exactly. If you added hän here (Jos hän nukkuu tarpeeksi, hän oppii paremmin), it would change the meaning to refer to a specific person: 'If he/she sleeps enough, he/she learns better.' Leaving the subject out completely is what makes it apply to anyone.

The word tarpeeksi ends in -ksi. Is that the translative case?

Yes, it originally comes from the noun tarve (need) in the translative case. However, in modern Finnish, tarpeeksi is just a fixed adverb meaning 'enough,' so you can just memorize it as a single vocabulary word.

Why is it paremmin instead of parempi?

Because it modifies the verb oppii (learns). Paremmin is the comparative adverb meaning 'better' (from hyvin = well). You would use the adjective parempi if you were describing a noun, like a better student (parempi opiskelija).