It was really lucky that Harry now had Hermlone as a friend. He didn't know how he'd have gotten through all his homework without her, what with all the last-minute Quidditch practice Wood was making them do. She had also lent him Quidditch Through the Ages, which turned out to be a very interesting read.
Harry learned that there were seven hundred ways of committing a Quidditch foul and that all of them had happened during a World Cup match in 1473; that Seekers were usually the smallest and fastest players, and that most serious Quidditch accidents seemed to happen to them; that although people rarely died playing Quidditch, referees had been known to vanish and turn up months later in the Sahara Desert.
(Harry Potter and the Sorcerer's Stone)
In a sentence, the first clause has past tense, and the second past perfect. What semantic difference comes from the different tenses?