Learn Finnish: Tämä on pomo, jonka ura on mahtava. - This is the boss whose career is awesome.

jonka is the genitive (possessive) form of the relative pronoun joka (who/which/that). It shows ownership, so it translates directly to "whose" in this sentence because the career belongs to the boss.

Does jonka get its singular form from pomo (boss) or ura (career)?

It gets its form from pomo (the owner). A relative pronoun always takes its number (singular or plural) from the word it refers back to. Because there is one boss, we use the singular genitive jonka. If we were talking about multiple bosses, we would use the plural genitive joiden, regardless of how many careers they have.

Why is there a comma before jonka?

In Finnish, punctuation rules are very strict about relative clauses. A clause starting with any form of joka must always be separated from the main sentence by a comma, unlike in English where it is sometimes optional.

What is the literal feeling of the word mahtava?

While it commonly translates to "awesome" or "great" in everyday speech, mahtava comes from the word for power (mahti). It gives a strong feeling of something being mighty, massive, or grand, rather than just being "nice".