Complex sentence: Originally, Mork's appearance was explained as a dream of Richie's, but after the spin-off was established, a new ending was tagged on to the repeat of the "Happy Days" episode explaining that Mork would return to Earth in 1978.
Split into simpler sentences: Originally, Mork's appearance was explained as a dream of Richie's. But after the spin-off was established, a new ending was tagged on to the repeat of the "Happy Days" episode explaining that Mork would return to Earth in 1978.