An American in Provence

Many languages, including french, have the informal/singular Tu form for you, and the formal/plural form Vous. One thing that is difficult as a foreigner to figure out is when to use each one. It seems that Americans feel strange with using Vous, especially to each other, as it feels snobbish and elitish.

Basically, as an adult, when first meeting someone or with a stranger (say a shopkeeper or someone you ask directions from in the street), you use Vous. But, sometimes if a friend presents you to one of their friends, you can use Tu right away. If you start off with Vous, then eventually, the other person might say "Ok, you can use Tu with me now." It is an act of friendship and recognition. There are even verbs for it, tutoyer and vouvoyer. Or if someone is being too familiar with you, you can say, "Oh, I am not your friend. You can vouvoyer me."
A webarticle on it
http://www.thewritefrenchlife.com/TuorVous.html
With children, you usually address them as Tu. Then they can address you as vous or tu. Students and teachers, it depends. French children have trouble to use Tu with their professors.

On a forum the question was posed

"if you were to switch to vous with someone with whom you usually tutoyer, would
it in a way be a ruder form, as if you are being cold and distant? For example, suppose that a friend of yours does something that really upsets you; could you
vouvoyer them to let them know that you are not happy with them?"

the general response seemed to be

"in my experience once you start saying tu to someone you never revert to
saying vous. "

and "Yes, it's quite a slap in the face actually and they'll wonder what they've done to you. Once you start 'tutoyer' someone there's no going back."

and oppositely

"I remember reading an interview of Brigitte Bardot (who comes from quite a
posh family) in which she told a story about her childhood: one day she and her
sister broke a precious vase and as a punishment her parents started to say
'vous' to both children on the spot. They never used 'tu' again. Little Brigitte
was absolutely distraught and never forgave them, even as an adult."

Sound confusing? Yes, it is. When in doubt, follow the french person's lead. Sometimes, I am not sure how the vous form is pronounced, because I usually learn the Je (I) form first, which usually sounds like the Tu form and the 3rd person singular (he, she) form. Thankfully, one can use the universal On- which can be you, me, a person in general, or us.