You see a person you work with in class in the hallway. They say "Hi". What do you do?
Say "Hi" back. Look in their direction and smile. Look in their direction and give a head nod.
How do you greet a person that you are friendly with, but you don't know very well?
Say "Hi". Ask how they are doing, but only expect a short answer.
How do you know if someone is your friend?
You share something in common, You can trust them, You feel comfortable being around each other, You agree to be friends, You spend time with them.