Zmarł trzy lata temu.

  • zemrzeć ‑mrę ‑mrzesz zmarł pf, impf zmierać = to die, pass away
  • umrzeć umrę ‑rzesz umarł pf, impf umierać = to die
  • Difference (as confirmed by Adam):-
    “Zemrzeć” is more formal than “umrzeć” – it’s kind of like the difference between “decease” and “die”.

Also, “zemrzeć” is mostly used in past tense (zmarł, zmarła, zmarli, etc.) – you’ll see it a lot in obituaries, encyclopaedic entries and so on. The infinitive (zmierać), present forms (zmieram, zmiera, zmierają, etc.) and future forms (zemrze, zemrą, etc.) are very rare, and sound somehow archaic.

On the other hand, “umrzeć” is commonly used in all tenses. For example, if you want to say “he is dying”, “on umiera” (and not: “on zmiera”) is pretty much your only choice.