"The Mongolian writer sits on the fiftieth chair."
Translation:A mongol író az ötvenedik széken ül.
It doesn't have to be at the end but you can't start the sentence with "ül". These are also correct:
A mongol író ül az ötvenedik széken.
Az ötvenedik széken ül a mongol író.
The word before the verb is emphasized. So DL's and my sentence emphasize that he sits on the 50th and not the 49th chair. My first sentence emphasizes that it's the Mongolian writer who sits there, not the Colombian doctor.
Why couldn't you start with "ül"?
"Ül az ötvenedik széken a mongol író." - perfectly fine.