*The title of this post is based off of something our adviser (Judith) said during a meeting during the prestage- I believe she said that it would make a good title for an article or something, so I took it literally and decided to make a post on it. I apologize if anyone has already made a post on this- and if I've now taken your idea, well; you had plenty of time to do it.*
France is referred to as a republic- meaning that it should have a social structure meant for everyone to be more or less equal, specifically in the government. I believe it has been like this since the French Revolution, where the citizens lobbed off the heads of Marie Antoinette and Louis the 16th- the "Absolute Monarchs" and made their own government.
What contradicts this idea of equality, however strangely enough, is the language itself. Unlike in English, there are two ways to say 'you': tu, and vous. Vous works for plural 'you,' as in the ever popular southernism 'y'all'. However, it is used also in the singular as a more proper, polite way of saying 'tu.' And people take the use seriously here.
The general rule of thumb that I would define it that if you can call someone by their first name only, one can use tu. All other cases, vous and all other polite forms (possessive pronouns, conjugations, commands, etc.: pretty much to be expected for the sake of consistency) should be used. One thing that people tend to forget, though, it Salut.
Salut translates pretty closely to 'hi,' in English, with Bonjour being the more formal "hello," but it is never used the same way. Salut is another one that you would only use with tu, so while in the United State, we go around saying 'hi' to everybody, if one tries to do the same thing here in France, it would end up with dirty looks, among other things possibly.
So, morale of the story; unless you know absolutely sure, use vous.