Complex sentence: Marty spots a tombstone with Doc's name, dated six days after the letter, learning that Doc was killed by Biff Tannen's great-grandfather, Buford "Mad Dog" Tannen.
Split into simpler sentences: Marty spots a tombstone with Doc's name, dated six days after the letter. Learning that Doc was killed by Biff Tannen's great-grandfather, Buford "Mad Dog" Tannen.