Throughout his time at Hogwarts, Harry had many less than pleasant dealings with Severus Snape. As a student, Harry felt like Professor Snape was out to make his life miserable, but it wasn't until after Snape's death that he learned to understand Snape's actions. Despite Harry's dislike of Snape as a professor, he couldn't help but pick up some valuable lessons because Snape was in fact an amazing instructor.
1. The practical use of magical resources
His classes may have been a challenge, but Snape was an excellent professor who taught his students applicable uses of potions, and the defense against the dark arts. Without Snape, Harry would have never known to use bezor to save Ron after he was poisoned. It was also Snape who first demonstrated the use of the disarming spell that would become Harry's go to defense.
2. Life isn't fair.
Every child should learn this at some point. We don't always get what we want, and if we do, it's probably because we had to work hard to get it. What's worse is that we usually have to sit by and watch others get exactly what they want without even trying because life isn't fair.
Snape has no tolerance for disrespectful sassy children. You must respect him as an authority and call him Sir or Professor.
4. Things don't always work out the way you planned.
Sometimes you have good intentions, but everything goes wrong and people you love get hurt.
4. Love is undying.
Snape is proof that the ones you love always hold a special place in your heart.
5. Duty means doing things you don't want to do.
Sometimes people ask too much of us, but sometimes it's for the greater good and we have to bite the bullet and do our duty.
6. Protect your mind.
Snape is a master at Occulmency which is a perfect skill to master for a double agent and a man who holds so many secrets. If only Harry paid more attention. I do hope he took the time to practice this when he became an Arour.
7. Keep your emotions in check.
By this point in Order of the Phoenix, Harry is dealing with the PTSD of Voldemort's return and Cedric Digory's death as well as average hormonal teenage outbursts, and Snape gives the best constructive advice.
8. Be suspicious of everyone.
Snape was the one who suspected Professor Quirrell of attempting to steal the sorcerer's stone. He is naturally suspicious of students because they are obviously up to something
9. Play it cool.
Snape is the best double agent. For the entire series, we go back and forth trying to figure out what side Snape is actually on.
10. Help will come whether you ask or not.
People like Severus Snape always have a watchful eye, and are always there when you need their services, but they don't expect or desire your gratitude.
11. Heroes don't always get recognition.
Harry grew up in the spotlight, and it wasn't until after Snape's death that he understood the extent of Snape's bravery.