Complex sentence: He studied musical theory with composer Gustav Fredrik Lange (1861-1939) during 1921-22 and during 1921-1925, he was a student of piano composer Nils Larsen (1888-1937).
Split into simpler sentences: He studied musical theory with composer Gustav Fredrik Lange (1861--1939) during 1921--22. From 1921 to 1925, he was a student of piano composer Nils Larsen (1888--1937).