Complex sentence: The impasse is resolved when Gilbert and his wife visit a popular exhibition of Japanese arts and crafts; when the katana sword he purchased there falls noisily off the wall of his study, he is inspired to write a libretto set in exotic Japan.
Split into simpler sentences: The impasse is resolved after Gilbert and his wife visit a popular exhibition of Japanese arts and crafts. When the katana sword that he purchased there falls noisily off the wall of his study, he is inspired to write a libretto set in exotic Japan.