Here are some spells that wizards use. The Patrounous charm,Lumoths,and Mischef maneged.

I believe I may have figured out Snape's greatest secret. It is a secret that explains everything: why Snape hates Harry Potter (above and beyond the fact that Snape hated his father), yet risks his life to save him, why Snape wants the Defense Against the Dark Arts position, yet never gets it, why Snape is utterly trusted by Dumbledore, and why Voldemort believes he can steal the power of Harry's mother's love by stealing Harry's blood.

I believe that Snape already saved Harry's life - when he was one year old. I think Snape defeated Voldemort.

Snape was spying for Dumbledore before the defeat of Voldemort, and kept his cover well enough that they could actually send him back in after Voldemort's return, to spy again in book five. However, this isn't enough to explain why Dumbledore is certain that Snape couldn't be a double agent, or willing to switch sides again, all by itself. There has to be more.

My theory comes in part from Snape's first introductory statements to his students in Book 1: "....I can teach you to bottle fame, brew glory, even stopper death...." I believe that Snape, knowing that Voldemort was after Harry Potter, offered to create a protective potion, by brewing Mother's Love. He may have offered this to stop Voldemort, to show up James Potter, and possibly because he loved Lilly. His potion infused Harry's mother's love into a powerful anti-curse enchantment, which settled into Harry's blood (as potions are known to do).

Snape wants recognition. He wants fame. He wants to let the entire world know that he, and he alone, has stopped the unstoppable curse. HE DEFEATED VOLDEMORT. Yet, he toils in ignominy, as a teacher of Potions, hardly the most respected of the Arts. To protect the secret of Snape's incredible achievement, Dumbledore won't even let Snape teach Defense Against the Dark Arts.

Then comes Harry Potter. Son of the hated James Potter. And HE GETS ALL THE CREDIT.

HE'S "The Boy Who Lived". HE'S famous. HE gets all the credit for stopping Voldemort, when he didn't do ANYTHING AT ALL. HE gets to break the rules, is beloved by half the wizarding world, is seen as a savior.
And all that fame rightfully belongs to Snape.

And Snape? He gets NOTHING. He can't even teach Defense Against the Dark Arts, thereby showing that he at least an expert in this prestigious area of expertise. Even to this day, people still accuse him of being evil, of continuing to work for Voldemort. Even that ungrateful Potter boy suspects him of being Voldemort's ally. And he can't even tell them the truth. In his own mind, at least, he is the greatest defender against the Dark Arts in the world, possibly even greater than Dumbledore himself, and he gets no respect for it at all.