Complex sentence: At the apartment, Barney is introduced to Marissa Heller, who Ted explains was their apartment's previous occupant and that they have been receiving her mail ever since they had moved in ten years ago.
Split into simpler sentences: At the apartment, Barney is introduced to Marissa Heller, who Ted explains was their apartment's previous occupant. Ted has have been receiving her mail ever since they had moved in 10 years ago.