Complex sentence: Kleiber made a successful Bayreuth debut in 1974 conducting Wagner's "Tristan und Isolde"; he led this opera again in 1975 and (for three performances only) in 1976, but did no further work at the festival.
Split into simpler sentences: Kleiber made a successful Bayreuth debut in 1974 conducting Wagner's "Tristan und Isolde". He led this opera again in 1975 and, for three performances only, in 1976, but did no further work at the festival.