Complex sentence: De Angelis retired from the operatic stage in 1939 and although he gave a few concerts during the early 1940s, he henceforth concentrated his activities on providing voice lessons to pupils in Milan and Rome.
Split into simpler sentences: De Angelis retired from the operatic stage in 1939. Although he gave a few concerts during the early 1940s, he henceforth concentrated his activities on providing voice lessons to pupils in Milan and Rome.