Complex sentence: Later he went to Posen, where he remained for several years, and at about 1641 he became rabbi of the old community of Ostrog, or Ostroh, in Volhynia.
Split into simpler sentences: Later he went to Posen, where he remained for several years. Around 1641 he became Rav of the old community of Ostrog, or Ostroh, in Volhynia.