Time-turners were only used in "Prisoner of Azkaban" to save Sirius, raising questions about their potential and limitations. J.K. Rowling addressed the time-turner plot holes by introducing limitations on time travel and ultimately destroying all time-turners.
Why didn't they use the Time-Turner to stop Voldemort?
These instances followed Rowling's stance on self-fulfilling prophecies. With the established Harry Potter time travel rules, Harry and his friends never had the option of traveling back in the past to kill Voldemort. If they were to do so, Voldemort would already be dead in the present timeline.
Why didn't they use Time-Turner in Goblet of Fire?
The time turner couldn't have been used to save Harry's parents because it was too far back in time; you cannot go back more than five hits without there being very bad consequences. They saved Buckbeak because he was about to die three hours before.
Going into their third year of academics, Hermoine and her friends had a lot of subjects to choose from. Hermione had more subjects than she could fit into her timetable. She was given the time Turner to attend more than one classes simultaneously, purely academic purposes.
Why didn't Hermione use the Time-Turner to save Sirius?
Changing the past is hazardous and apparently nigh impossible. Buckbeak could be rescued because he already had been rescued, while Sirius was seen to have died by several witnesses. Even if it were possible, Hermione only had the Time Turner for one year.
Why did McGonagall trust Hermione with the Time-Turner?
Perhaps McGonagall saw Hermione's potential and knew that someone like her could show that the Time-Turner had applications for students looking to take on extra credit at school or, in the case of Prisoner of Azkaban's finale, save innocent people from evil.
Hermione explains that the hourglass is a Time-Turner; she has been using it all year to get to all her classes. Professor McGonagall instructed her to tell no one about it.
The user might still age while within an hour produced by the Time-Turner, and if so, then for wizards or witches such as Hermione, constant use might age them faster, adding days, weeks, or even months onto their internal chronological clock. It is also unknown how this applies to later on in life.
The presence of time-turners ended up creating big plot holes in the Harry Potter saga, as these could have been used to prevent some of the biggest tragedies in the series, mainly the death of Harry's parents.
Honestly, Hermione knew Dobby was a lost case... nothing could have revived Dobby after the attack or she would have definitely helped Harry with Dobby considering she was the girl who started S.P.E.W(Society for the Elfish welfare) and fought for elf rights. There was nothing to be done at that point.
Dumbledore's lack of belief that Sirius is innocent is also suspect. Sirius was a member of the Order of the Phoenix which fought against Death Eaters, but Sirius also was as close to James as a brother, the Potters even took him in when he ran away from home.
Hermione found her third year stressful with the extra class load, and therefore decided to drop Divination, which she despised, and Muggle Studies, which she did not find very useful, given that she was a Muggle-born. This allowed her to have a normal schedule once again, and she returned her Time-Turner.
Dumbledore first stresses the importance of Hermione and Harry being unseen, and then later Hermione tells us that witches and wizards who have meddled with time carelessly have caused terrible damage, sometimes even killing their past or future selves and thereby completely destroying the semblance of logic within ...
After the Mintumble incident, the Department of Mysteries terminated such experiments, and Ministry of Magic laws dramatically restricted the use of time-turners.
Hermione signed up for all 3rd Year Electives: (Arithmancy, Care of Magical Creatures, Divination, Muggle Studies, & Study of Ancient Ruins) Hermione had a total of 12 classes & has a time turner. Chapter 22 - Hermione can drop 2 classes (Divination & Muggles Studies) and return to a normal schedule (10 classes).
Hermione Granger took Muggle Studies, as well as every other class, in her third year, despite being Muggle-born. She wanted to study Muggles from the wizards' point of view, and excelled in the class, but dropped it at the end of the year when she found her schedule too demanding.
She and her husband were proud and accepting of their daughter's identity as a witch, which explained some strange things which happened around her when she was a child. They entered the magical world in 1992 to buy Hermione's school supplies, and were assisted by Arthur Weasley.
According to The Leaky Cauldron, when a fan asked if Hermione made sure that her parents could remember her again, Rowling said, "she brought them home straight away." This is good news, as it's awful to imagine Hermione's parents never remembering who she is or ever seeing her again.
The author also said the charm wasn't a true Obliviate spell. Hermione actually bewitched her parents to believe they were different people. In either case, Hermione Granger and her parents reunited. According to Rowling, they continue to be proud of their daughter's accomplishments.
Thinking about her future, she realised that wedding a Muggle who didn't know that she was a witch would mean Minerva would follow her mother's footsteps and live unhappily with her wand locked away, shredding all of her ambitions.
Other than Pansy Parkinson with absolute zero room-reading skills and Professor McGonagall sending all the Slytherins to the dungeons, from what I understand McGonagall literally had them locked up in the dungeons cells than give them a choice whenever to fight against Voldemort or remain in the common room, because ...
Therefore, Snape could have accused Harry and Hermione that they used it to go back in time to free Sirius if he'd known about the time-turner. So the only teachers who knew about it would be Dumbledore and McGonagall.
