Why does Harry see his parents during the duel in the Triwizard Tournament?

Answer 1

The Mirror of Erised shows your hearts deepest desire. Harry Potter grew up without his parents and was raised by his Aunt and Uncle who weren't interested and mentally abused him. When Harry looks in the mirror he sees his whole family because he wants to experience being part of a family.

Answer 2

Harry's and Voldemort's wands are brothers as they contain twin cores given by Fawkes, Dumbledore's phoenix.

During the duel between them, the effect "Priori Incantatum" occured due to the wands identifying each other as brothesrs and refusing to destroy each other. As a result, Voldemort's wand discharged, in sequence, the last spells it had cast, as an attempt to equal the power difference between Harry and Voldemort, as at that time Voldemort was much more powerful than Harry, and could have easily overpowered his "Expelliamus"spell.

The spell "Priori Incantato" was cast to identify that Harry's wand was used to cast "Morsmordre" (the dark mark) during the Quidditch World Cup. This showed that the spell and effect shows the previous spell(s) cast by a wand in reverse sequence to when they were cast.

The reason for Harry seeing his parents and Cedric Diggory, as well as the muggle groundskeeper, was because the wand discharged the last spells it had cast. Which excluding "Crucio", were all "Avada Kedavra". The manifestations of Harry's parents were actually echos of the time when the spell was cast.

The way I understand it is that they were like ghosts, as it was described by Snape and Sir Nicholas, that ghosts are echos of the person's soul that roam the earth after death. This would explain how the manifestations were consciously able to speak and act against Voldemort, except they departed after the effect had ended as they weren't true ghosts.

Answer 3

Ron never saw the grave, only Harry and Hermione did.

They went to Godric's Hollow hoping for some answers in their search for the Horcruxes. Harry also wanted to see the place where his parents were killed. In the cemetry they found Lily and James grave.

Answer 4

Harry uses the Ressurection Stone to bring back imprints of the dead