Complex sentence: In 1996, Burstein met and talked with the Estonian composer Arvo Pärt, who was subsequently to become a mentor and who was instrumental in gaining Burstein a commission to write music for the nine-hundredth anniversary celebrations of Norwich Cathedral.
Split into simpler sentences: In 1996, Burstein met and talked with the Estonian composer Arvo Pärt, who was subsequently to become a mentor. Pärt was instrumental in gaining Burstein a commission to write music for the nine-hundredth anniversary celebrations of Norwich Cathedral.