Complex sentence: During the Music of the Ainur, Aulë's themes concerned the physical things of which Arda is made; when Eru (Ilúvatar) gave being to the themes of the Ainur, his music became the lands of Middle-earth.
Split into simpler sentences: During the Music of the Ainur, Aulë's themes concerned the physical things of which Arda is made. When Eru (Ilúvatar) gave being to the themes of the Ainur, his music became the lands of Middle-earth.