Snape and lily dating
While stopping at the Hog's Head pub, he eavesdropped on Dumbledore's interview with new Divination professor Sybil Trelanwey in the restaurant inn, during which Trelanwey prophesied of a child born at the end of July who would be marked as Voldemort's equal and cause the dark wizard's destruction (which prompted Dumbledore to hire her, partly for her own safety).
Snape did not hear the entire prophecy, however, as Dumbledore's younger brother Aberforth, who worked as the restaurant's barman, caught him and threw him out.
On one occasion, he noticed that Lupin kept disappearing during the full moon and followed him into the tunnel under the Whomping Willow planted on the school grounds, having been tipped off by Black on getting past the tree's branch defenses.
When Potter found out what happened, he went after Snape and saved him by pulling him back, but not before Snape glimpsed Lupin transforming into a werewolf in the supposedly haunted Shrieking Shack at the other end of the tunnel.
Although his information on the prophecy was incomplete, Snape reported to Voldemort what he heard.
Severus Snape, also known as the Half-Blood Prince, is a major character of the Harry Potter franchise.
Dumbledore swore all four boys (Snape, Potter, Black and Pettigrew) to keep such discovery about Lupin a secret, but Lupin's supposed true nature was clear to Snape, who in addition believed Potter's actions were self-serving to avoid expulsion.
On one such occasion in his fifth year, Snape tried to run from the Marauders after finishing his homework under a tree on the Hogwarts grounds, but Potter hoisted him upside-down to reveal his underwear in front of many students.
While Lily was sorted into Gryffindor due to her Muggle-born status and courage, Snape ended up in Slytherin due to his resourcefulness with the Dark Arts and ambition from the abuse given by his father.
While there, he made good rapport with Slytherin prefect Lucius Malfoy as the two grew to share their extremist ideals, and eventually became part of a small gang of Slytherins (consisting of Rabastan Lestrange, Evan Rosier, Gaspar Avery, Adam Mulciber and Wilkes) which embraced dark magic and pure-blood supremacy, the views and qualities of which even Lily was offended by since it marked Muggle-borns, half-bloods, blood traitors and even half-breeds disgraceful.