Complex sentence: Marty and Doc determine that the 2015 Biff took the time machine to change 1985, and Marty learns from the alternate 1985 Biff that he got the almanac on November 12,1955.
Split into simpler sentences: Marty and Doc determine that the 2015 Biff took the time machine to change 1985. Then Marty learns from the alternate 1985 Biff that he got the almanac on November 12,1955.