Professor Lupin, the new Defense Against the Dark Arts teacher, is the first Defense Against the Dark Arts teacher in Harry's experience to demonstrate knowledge and mastery of the subject, as well as the first to take on the role of mentor for any of his students. While his predecessors were either ineffective frauds or seemed terrified of the subject, Lupin presents his students with lessons that follow a logical progression through age-appropriate material and, at the end of the school year, leave them well-prepared to take on any of the nefarious creatures they learned about. Through Lupin, then, the novel is able to explore what makes a good teacher, especially by comparing him with Professor Snape, the potions master who's known for playing favorites and terrorizing students he doesn't like. With this, the novel suggests that what sets good teachers apart from middling or bad teachers is a genuine interest in students as people and the ability to instill confidence in their pupils.
Lupin earns the respect of Gryffindor students before they even arrive at Hogwarts. When dementors board the Hogwarts Express to search the train, Lupin doesn't allow them to terrorize the students in his car and after they leave, he distributes chocolate (an antidote to the dementors). This is an indicator that he not only knows what he's doing and knows how to deal with dementors, it shows that he also believes it's his responsibility to care for the students' health and wellbeing. Even more importantly, Lupin demonstrates during the third-year Gryffindors' first lesson with him that he's aware of his students' quirks and social standings, as he uses what he knows to tailor the lesson to their needs. This is why he asks Neville to assist him with the boggart at first--he knows that Neville isn't at all confident in his abilities, but also knows that if given encouragement, Neville is capable of succeeding. Then, by not allowing Harry to face the boggart to prevent an incarnation of Voldemort frightening everyone in the classroom, he also demonstrates the ability to plan lessons that won’t completely terrify his students. For Harry’s conception of Lupin as a competent and caring teacher solidifies even more when Lupin, confronted with Harry's question of why he wasn't allowed to take on the boggart, doesn't deny that he purposefully denied Harry the chance. This impresses upon Harry that though Lupin is an adult and an authority figure, he also recognizes that his students are people who are just as deserving of having their questions answered truthfully as anyone else.
If Lupin is construed as the living embodiment of good teaching, Snape represents the exact opposite. Potions lessons are dismal and anxiety-inducing affairs for anyone who isn't a Slytherin student, especially if Snape already dislikes them. During his classes, Snape regularly exhibits unethical behavior. He tries to poison Neville's toad, Trevor, in their first Potions lesson, exhibits blatant favoritism that affects students' grades, and verbally abuses the students when they either fail to properly concoct his potions or, as with Hermione, offend him by demonstrating mastery and understanding of the subject while also being a member of a House he doesn't like. Though this all explains why none of the Gryffindors do particularly well in Potions classes, Snape's teaching methods also call into question whether or not his favored Slytherins do well because they actually understand the material or if they succeed just because they're favorites. Even though the Slytherins are considered "bad guys" throughout the series, that doesn't mean they're exempt from Hogwarts's entire mission: to prepare students for adult life in the wizarding world, something that, presumably, requires a working knowledge of potion-making.
The differences between Lupin and Snape's teaching styles become most apparent in the Shrieking Shack. The high emotional tenor of the situation means that Snape lets his emotions and biases get the better of him, and he screams at Hermione several times throughout the evening. Lupin, on the other hand, treats Hermione and Harry's questions as valid and worthwhile, and he does nothing without asking the students' permission and consent. Taken together, Lupin's behavior towards Harry, Ron, and Hermione means that they're able to feel a sense of involvement and ownership over the information they learn in the Shrieking Shack, in much the same way that Lupin encourages them to think about their lessons in school. Snape's behavior, however, makes it clear that he doesn't value students as people: the trio, students he's supposed to be in charge of protecting and nurturing, are nothing more than an obstacle standing between him and revenge for a childhood grudge, as well as the promised honor of the Order of Merlin for catching Sirius Black.
In many ways, what Harry, Ron, and Hermione learn about teaching during their third year at Hogwarts serves to shape their ideas about teaching in the future and, in particular, deepens their hatred and distrust of Snape. Lupin's example provides Harry the framework to later go on to teach his classmates lessons of his own design, while Lupin himself remains a mentor to the trio throughout the series. However, this does offer insight into the power of teachers on a much broader scale: while a good teacher can have lasting positive effects on former students for years to come, teachers who don't treat their students fairly not only damage students' self-esteem, but they also fail to prepare them for life in the real world by making it so students leave school without the tools or knowledge they need to succeed.
Teaching Quotes in Harry Potter and the Prisoner of Azkaban
Non-magic people (more commonly known as Muggles) were particularly afraid of magic in medieval times, but not very good at recognizing it. On the rare occasion that they did catch a real witch or wizard, burning had no effect whatsoever.
"The guards told Fudge that Black's been talking in his sleep for a while now. Always the same words: 'He's at Hogwarts...he's at Hogwarts.' Black is deranged, Molly, and he wants Harry's dead. If you ask me, he thinks murdering Harry will bring You-Know-Who back to power. Black lost everything the night Harry stopped You-Know-Who, and he's had twelve years alone in Azkaban to brood on that."
"Possibly no one's warned you, Lupin, but this class contains Neville Longbottom. I would advise you not to entrust him with anything difficult. Not unless Miss Granger is hissing instructions in his ear."
"Why didn't you let me fight it? said Harry abruptly.
Lupin raised his eyebrows.
"I would have thought that was obvious, Harry," he said, sounding surprised.
Harry, who had expected Lupin to deny that he'd done any such thing, was taken aback.
"Didn't they want to help, sir?" said Percy.
"Oh yes," said Dumbledore coldly. "But I'm afraid no dementor will cross the threshold of this castle while I am headmaster."
"Anyone?" Snape said, ignoring Hermione. His twisted smile was back. "Are you telling me that Professor Lupin hasn't even taught you the basic distinction between--"
"We told you," said Parvati suddenly, "we haven't got as far as werewolves yet, we're still on--"
"Silence!" snarled Snape.
"But you were innocent!" said Hermione.
"Think that matters to them? They don' care. Long as they've got a couple o' hundred humans stuck there with 'em, so they can leech all the happiness out of 'em, they don' give a damn who's guilty an' who's not."
"THE DARK LORD LIES ALONE AND FRIENDLESS, ABANDONED BY HIS FOLLOWERS. HIS SERVANT HAS BEEN CHAINED THESE TWELVE YEARS. TONIGHT, BEFORE MIDNIGHT...THE SERVANT WILL BREAK FREE AND SET OUT TO REJOIN HIS MASTER. THE DARK LORD WILL RISE AGAIN WITH HIS SERVANT'S AID, GREATER AND MORE TERRIBLE THAN EVER BEFORE."
"But if--if there was a mistake--"
"KEEP QUIET, YOU STUPID GIRL!" Snape shouted, looking suddenly quite deranged. "DON'T TALK ABOUT WHAT YOU DON'T UNDERSTAND!"
"Of course," Lupin breathed. "So simple...so brilliant...he cut if off himself?"
"Just before he transformed," said Black. "When I cornered him, he yelled for the whole street to hear that I'd betrayed Lily and James. Then, before I could curse him, he blew apart the street with the wand behind his back, killed everyone within twenty feet of himself--and sped down into the sewer with the other rats..."
"Yes, I do," said Dumbledore quietly. "But I have no power to make other men see the truth, or to overrule the Minister of Magic..."
Harry stared up into the grave face and felt as though the ground beneath him were falling sharply away. He had grown used to the idea that Dumbledore could solve anything. He had expected Dumbledore to pull some amazing solution out of thin air. But no...their last hope was gone."
"Did anyone see you?"
"Yes, haven't you been listening? I saw me but I thought I was my dad! It's okay!"
"It didn't make any difference," said Harry bitterly. "Pettigrew got away."
"Didn't make any difference?" said Dumbledore quietly. "It made all the difference in the world, Harry. You helped uncover the truth. You saved an innocent man from a terrible fate."