Complex sentence: The main theme, in octaves, is quiet and ominous; it consists of a down-and-up arpeggio in dotted rhythm that cadences on the dominant, immediately repeated a semitone higher (in G flat).
Split into simpler sentences: The main theme, in octaves, is quiet and ominous. It consists of a down-and-up arpeggio in dotted rhythm that cadences on the dominant, immediately repeated a semitone higher (in G flat).