What's the difference between здравствуйте, привет, and здравствуй? When is it appropriate to use each one? Is one more formal than the rest?
"Здравствуйте" is a polite/formal choice to say "hello". It is usually used with strangers, old people.
"Здравствуй" is an informal one for "hello". It"s used with friends, relatives.
"Привет" is the most informal choice, it coincides with "hi" in English. It is used the same like the "здравствуй" with friends and relatives.
That's not the best approach. It may be OK to say hi to someone you don't know in English, but it's totally not OK to say привет to a stranger in Russian. Perhaps among people of the same age, say at a university, but not to a clerk when entering a store, for instance, not to your friend's parents and not to an older person.
I would recommend sticking to здравствуйте, добрый день/вечер or the equivalents by default, unless you are sure you are at the level of familiarity with a person that allows for здравствуй/привет/как дела?/and so on.