Complex sentence: It retained this title until 1978 when it was renamed the Théâtre National de l'Opéra de Paris "; in 1989 it was renamed again as the" Palais Garnier ", after the Opéra Bastille" was completed and became the opera company's principal theatre.
Split into simpler sentences: It retained this title until 1978 when it was renamed the "Théâtre National de l'Opéra de Paris". In 1989 it was renamed again as the "Palais Garnier" and, after the "Opéra Bastille" was completed, it became the opera company's principal theatre.