Complex sentence: Prior to the start of Day 1, Nina had an affair with Jack during his separation from his wife Teri Bauer, which ended with Jack and Teri's reconciliation and Nina's romantic involvement with colleague Tony Almeida.
Split into simpler sentences: Prior to the start of Day 1, Nina had an affair with Jack during his separation from his wife Teri Bauer, which ended with Jack and Teri's reconciliation. At the start of Day 1, Nina is romantically involved with colleague Tony Almeida.