Complex sentence: In 1991, they first fought for the WWF Championship during two different pay per view events, with the first being the Survivor Series event, where Undertaker defeated Hogan to win his 1st WWF title.
Split into simpler sentences: In 1991, they first fought for the WWF Championship during two different pay per view events. At the Survivor Series event, Undertaker defeated Hogan to win his 1st WWF title.