Harry remembered only too well the occasion where Ron's old wand had snapped. It had happened when the car the two of them had been flying to Hogwarts crashed into a tree on the school grounds. (1.48)
We get a lot of sentences like this throughout the novel, in which Harry recalls things that have happened in the previous jam-packed years at Hogwarts. While this could be seen as regularly scheduled visits by the Exposition Fairy (little details thrown in to get everyone up to speed), we think that these sentences have a deeper meaning. They help to reinforce how this novel functions as a meditation on the past.
"Anyway, they cornered Black in the middle of a street full of Muggles an' Black took out 'is wand and 'e blasted 'alf the street apart, an' a wizard got it, an' so did a dozen Muggles what go in the way. 'Orrible, eh?" (3.73)
It's interesting that Stan speaks of Sirius's actions as something he has first-hand knowledge of, even though he was a child at the time (Stan is described as someone in his late teens). The massacre Sirius committed has become a collective memory, or a big event that everyone in the wizarding community "remembers," kind of like how Americans "remember" events like Pearl Harbor or the fall of the Berlin Wall, regardless of whether or not they were there, or were even alive, at the time.
He knew he was being stupid, knew that the Nimbus was beyond repair, but Harry couldn't help it; he felt as though he'd lost one of his best friends. (10.1.1)
Harry has so many memories of his Nimubs that it's become a sort of "friend" to him. And as anyone who's seen Toy Story knows, inanimate objects can indeed be really great friends.
When the Dementors approached him, he heard the last moments of his mother's life, her attempts to protect him, Harry, from Lord Voldemort, and Voldemort's laughter before he murdered her [...] Harry dozed fitfully sinking into dreams of clammy, rotted hands and petrified pleading, jerking awake to dwell again on his mother's voice. (10.1.5)
In terms of style, Azkaban generally uses fairly short sentences and features lots of dialogue. But we get occasional passages like this, where there are a series of clauses that help to emphasize emotion. Here, we get a bunch of descriptions of what Harry experiences around the Dementors, which emphasizes how his experience is a complex, emotional, and ongoing affair.
"It has nothing to do with weakness," said Professor Lupin sharply, as though he had read Harry's mind. "The Dementors affect you worse than the others because there are horrors in your past that others don't have." (10.2.31)
Lupin introduces one of the major themes of the book here, as well as an important lesson to Harry – being affected by the bad parts of your past isn't a weakness. And being affected by the past is actually a good thing, since the past shouldn't be ignored. Harry wouldn't be the person he is if he could just brush the Dementors off and go about his day.
He stopped on the picture of his parents' wedding day. There was his father waving up at him, beaming, the untidy black hair Harry had inherited standing up in all directions. There was his mother, alight with happiness, arm in arm with his dad. And there [...] that must be him. Their best man [...] Harry had never given him a thought before. (11.1.4)
We see a lot of different "artifacts" from the past in this novel, turning the whole thing into an episode of History Detectives at times. It's worth noting that the past leaves concrete relics behind.
"And how do you conjure it?"
"With an incantation, which will work only if you are concentrating, with all your might, on a single, very happy memory." (12.3.11-12)
While the past is often bad in this book, Harry does have happy memories too. It's rather poetic that, to produce a Patronus, you have to focus on the good times even while surrounded by darkness.
Terrible though it was to hear his parents' last moments replayed inside his head, these were the only times Harry had heard their voices since he was a very small child. But he'd never be able to produce a proper Patronus if he half wanted to hear his parents again [...] (12.3.76)
Poor orphan Harry – we feel a lot of sympathy for our hero here. This passage emphasizes how long it's been since Harry has heard his parents' voices. It's fitting that he can't produce a Patronus (i.e., create a spell that depends on having a strong happy feeling) while he's clinging to his tragic past.
"Oh no," said Lupin. "Much worse than that. You can exist without your soul, you know, as long as your brain and heart are still working. But you'll have no sense of self anymore, no memory, no [...] anything. There's no chance at all of recovery. You'll just – exist. As an empty shell." (12.5.22)
The Dementors are like some sort of existential nightmare come to life – they can force people to wander around as empty shells, "existing" physically but not mentally. It's significant that Lupin points out "memory" as a key component to the "self." Even though dwelling on or obsessing over the past is a bad idea, no one should deny or totally forget what came before. It's a tricky balance that Harry has trouble with the entire novel.
"He bit Goyle for us once!" Ron said miserably. "Remember, Harry?"
"Yeah, that's true." said Harry.
"His finest hour," said Fred, unable to keep a straight face. "Let the scar on Goyle's finger stand as a lasting tribute to his memory." (13.1.9-11)
Though this scene is really funny, it also points to an important theme in the novel: people's tendency to recall the past fondly and to gloss over what it actually was. In Scabbers's case, that would be useless and boring. What other characters are fans of nostalgia and selective memory in the book?
"I met him!" growled Hagrid. "I musta bin the last ter see him before he killed all those people! It was me what rescued Harry from Lily an' James's house after they was killed! [...] an' Sirius Black turns up, on that flyin' motorbike he used ter ride. Never occurred to me what he was doin' there. I didn' know he'd been Lily an' James's Secret-Keeper." (10.3.160)
Hagrid undergoes an interesting experience here, where he learns that his memory of a past event wasn't what it seemed. Harry and his friends learn the "truth" about Sirius Black, and then learn that said "truth" was actually a lie, over the course of the novel. This makes for some unsettling experiences for the gang, as they confront the fact that memory and the past aren't stable or set in stone.
"A thought that still haunts me," said Lupin heavily. "And there were near misses, many of them. We laughed about them afterwards. We were young, thoughtless – carried away with our own cleverness." (18.50)
Interesting word choice here: Lupin speaks "heavily" about his past regrets, as if his guilt and anxiety are literally weighing down on him.
"This house" – Lupin looked miserably around the room, – "the tunnel that leads to it – they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform." (18.36)
The detail about Lupin looking around the room is notable – we can see him actually experiencing his past as he looks around the room.
"Sirius Black showed he was capable of murder at the age of sixteen," he breathed. "You haven't forgotten that, Headmaster? You haven't forgotten that he once tried to kill me?"
"My memory is as good as it ever was, Severus," said Dumbledore quietly. (21.61-62)
Poor Snape. OK, the guy is a total punk most of the time, but you can really hear his desperation here. The emphasis he places on "me" in his last question to Dumbledore is really telling. Snape wants Dumbledore to acknowledge him, and to acknowledge the past as Snape remembers it. Dumbledore's response is classic Dumbledore – vague and cryptic. But we think his "quiet" tone and his emphasis on his own "memory" shows that he's acknowledging Snape's concerns but without taking sides.
He was thinking about his father and about his father's three oldest friends [...] Moony, Wormtail, Padfoot, and Prongs [...] Had all four of them been out on the grounds tonight? (21.259)
We love the fact that this night of reunions and numerous bombshells occurred in the midst of a time-travel adventure – the past and the present really were brought together here.
"Harry," said Lupin hurriedly, "don't you see? All this time we've thought Sirius betrayed your parents, and Peter tracked him down – but it was the other way around, don't you see? Peter betrayed your mother and father – Sirius tracked Peter down." (19.69)
Ow, our brains. Lupin places emphasis on Peter's name here, which is significant. By stressing Peter, Lupin sort of drags him into the spotlight, trying to force the others to see Peter for the traitor he is with the power of logic and rhetorical strategy. Rhetorical strategy is a fancy-pants way of saying that Lupin is speaking carefully here. He emphasizes Peter's name with a specific intent, not just for the heck of it.
"You fool," said Lupin softly. "Is a schoolboy grudge worth putting an innocent man back inside Azkaban?" (19.7)
Snape is sort of the poster child for grudges. He refuses to let go of his own past and, as a result, things go rather spectacularly awry for him. Lupin's word choice really belittles Snape too. Lupin references Snape's "schoolboy grudge," which implies that Snape's feelings about Sirius are childish.