Complex sentence: Horacio does not enjoy having Rocamadour in his and La Maga's flat, and one afternoon, he decides that because Ossip so frequently explains his philosophical quandaries to La Maga, the two of them must be having an affair.
Split into simpler sentences: Horacio does not enjoy having Rocamadour in his and La Maga's flat. One afternoon, Horacio decides that because Ossip so frequently explains his philosophical quandaries to La Maga, the two of them must be having an affair.