Carolyn V. sent the following wink at the perils of confusing Tu and Vous:
http://www.latimes.com/opinion/op-ed/la-og-bastile-vous-tu-20140711-htmlstory.html
The dangers are serious! Already at 21, I made a peer feel that I was presumptuous when I said “tu” to a good friend of a close friend of mine – both French, of course. I heard about it the next day!
Another put-down was when a “tu” slipped out of my mouth to a younger French woman I only knew professionally. Her response: “On n’a pas élévé les vaches ensemble!” Quelle horreur!
I got a reprimand from Bobby, a high school student of mine who, in France for the summer, must have addressed an aristocrat as “tu.” Why hadn’t I told my students about the “you” problem, he complained in a post card. Of course, I had – in Lesson 1 – but he was probably thinking of something else then.
No matter how long we American democrat people know the rules, though, we can never feel it the way the French do.