Complex sentence: From 1920 to 1940, Eller was a professor of music theory and composition at the Tartu Higher School for Music, during which time he formed the Tartu school of composition which gave rise to many composers, including Eduard Tubin.
Split into simpler sentences: From 1920 to 1940 Eller was a professor of music theory and composition at the Tartu Higher School for Music. During this time he formed the Tartu school of composition, which gave rise to many composers, including Eduard Tubin.