After witnessing Nagini kill Severus Snape, Harry collected a string of memories from the dying man. During a one hour ceasefire, Harry discovered that he was an unintended Horcrux from Snape's memories. In order to destroy Voldemort, he had to allow the Dark Lord to kill him.
He probably got to know Harry was a Horcrux when he saw the Scar on Harry's forehead, but maybe he wasn't sure. Then, when Harry came to Hogwarts and Dumbledore got to know he can speak Parseltongue his thought of him being a Horcrux got stronger.
Notably, Voldemort appeared to be entirely unaware of this split, as he later created a seventh Horcrux in his quest to have six, and continued to target Harry Potter despite him now holding part of Voldemort's own soul.
IT was when Harry and Hermione were on their own without Ron, and were hunting for the Horcruxes, its when Harry used to wake up with terrible headaches in themiddles of the night screaming sometimes. Hermione's research into Horcruxes at that time concluded Harry was Voldermot's Horcrux.
Technically, Harry wasn't the 7th Horcrux, like most people think. Dumbledore explains that when the killing curse rebounded and hit Voldemort, another piece of his soul broke off and attached itself to the nearest living creature, Harry.
Why Didn't Voldemort Realize Harry Was a Horcrux? - Harry Potter Theory
Why didn't Harry use Avada Kedavra on the Horcruxes?
To Harry, Voldemort was the epitome of evil, as was the Killing Curse by extension, so he would not use the Unforgivable Curse that killed his parents. It's also believed that Avada Kedavra requires the caster to be willing to commit murder for the spell to work.
But Harry didn't die, and the reason is rather simple: when Voldemort cast the killing curse, it only killed the piece of his soul within Harry, not Harry himself. Harry then saw Dumbledore in a version of King's Cross station, something that Dumbledore said was probably happening in his head.
But the whole thing was a misunderstanding. No, Sirius Black was not involved in any horcrux business. It seems no one but Snape and Dumbledore were aware of this fact.
Family. Snape's family background is mostly shown in flashbacks during the course of the last three novels. Snape was born to Eileen Prince, a witch, and Tobias Snape, a Muggle, making him a half-blood (hence the name, "Half-Blood Prince").
Why was Hermione never affected by the Horcrux like Ron and Harry?
The only logical explanation for this is that Hermione didn't exhibit obvious weaknesses like Harry and Ron. They were unprepared and doubtful from the beginning, while Hermione was prepared for almost every scenario. The moment Hermione erased her parents' memories, she committed herself to the cause.
The prophecy is mentioned throughout the series, however, it never exclusively mentions that Harry is The Chosen One and suggests Neville was another option. However, one theory explains why, in reality, this wasn't possible.
On confirming Draco's very much alive status, she plainly lied that Harry Potter was "dead". This action of hers was both very crucial and important, not only for Harry's survival but for Voldemort's final defeat as well, thus ensuring Harry was again protected by a mother's love, although indirectly.
This would bring him into conflict with fanatical Death Eaters like Bellatrix Lestrange later in life, but Snape was almost immediately forgiven by the reborn Voldemort, because he could provide him with thirteen years of information on Dumbledore and the Order of the Phoenix.
Did Voldemort realize Harry was a Horcrux after he killed him?
Voldemort wasn't aware of Harry being a pseudo-horcrux because he didn't plan it, and Harry didn't know either until Voldemort “killed” him, but he actually killed the piece of soul kept in him.
Why didn't Dumbledore tell Harry how to destroy Horcruxes?
Dumbledore Never Really Had Time To Explain How To Destroy The Horcruxes. As much as Harry would have benefited from Dumbledore's knowledge of how to destroy the remaining five horcruxes, there was simply not enough time before his death to do so.
In the Harry Potter and the Deathly Hallows movie, Snape showed how much he loved Harry, providing him with his memories and showing that he saw Lily in him, his last words echoing this: "You have your mother's eyes."
The movies follow the books closely enough that they aren't really separate universes, except for the fourth movie. There isn't any truly compelling evidence that Snape is Harry's father, because all the points you raise are explained. Snape hated Harry because Harry reminded him of the man who "stole" Lily.
Under the influence of her father, Dolores grew up despising her mother and brother for their lack of magical abilities. She was sorted into Slytherin House at Hogwarts School of Witchcraft and Wizardry and hated her time at the school due to never being given any positions of power.
After Sirius died in 1996, his godson Harry Potter inherited all of Sirius Black's properties, including his house, his house-elf and his vault, which still contained "a reasonable amount of gold".
The clue he received in the film was from Bellatrix, who was in a frantic frenzy when she thought her vault was broken into. He is ultimately able to identify the cup as a Horcrux because of his ability to hear the piece of Voldemort's soul contained within them, an ability which is not present in the books.
Draco, who implies that he is aware of how Sirius Black was supposedly involved in the deaths of Harry's parents, also taunts Harry about the impending threat of Black: "If it was me, I'd want revenge. I'd hunt him down myself."
The two spells collided with a bang like a cannon blast — though it was short-lived. The Elder Wand, however, refused to harm its true master and flipped over, flying high toward its rightful master. The rebounding Killing Curse hit Lord Voldemort, and his lifeless body hit the floor, his white hands then empty.
Being a horcrux simply means that Harry had a piece of Voldermort's soul inside of him. That would not grant him immortality. Harry Potter after this part he is no longer a Horcrux, Voldemort destroyed his part of the soul that resided in the boy.
