We don't know for certain, but perhaps Dumbledore taught it to him.
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Harry Potter reminds Snape of Harry's father, with whom Snape had some trouble as a student.
John Nettleship, Rowling's chemistry teacher and the character on whom Snape is based, taught at Wyedean school near Chepstow.
Snape's patronas takes after Lily Potter's because Snape was in love with her.
the " snape, snape, severus snape, DUMBLEDORE!!" song is a youtube video. to see it type in, Potter Puppet Pals and The Mysterious Ticking Noise. there are also many more PPP (Potter Puppet Pals) videos but i cant name them all. :( sorry
Professor Snape uses the Avada Kedavra curse against him at the end of The Half-Blood Prince. This was a pre-arranged deal between Snape and Dumbledore, as we learn in Harry Potter and the Deathly Hallows-Dumbledore preferred that Snape kill him since he knew death was coming and he wanted it to be painless. He did not want to have to deal with getting attacked by Fenrir Greyback or getting tortured by Bellatrix Lestrange.