Original: He subsequently sang at Milan's La Scala (1951), the Glyndebourne Festival (from 1954 on) and London's Royal Opera House, Covent Garden (1954, 1956 and 1958).
Simplified: Then he sang at Milan's La Scala (1951), the Glyndebourne Festival (from 1954 on) and London's Royal Opera House, Covent Garden (1954, 1956 and 1958).