Complex sentence: In his youth, he mastered piano, guitar, violin, and flute, as well as the slide trombone, which at first he played in secret, as his conservative religious father believed it an uncouth and sinful instrument.
Split into simpler sentences: In his youth he mastered piano, guitar, violin, flute, and slide trombone. He kept his trombone activities a secret at first, as his conservative religious father believed it an uncouth and sinful instrument.