Quote Originally Posted by DDT
Yeah! I don't exactly know when to switch to informal. A Russian guy showed up in my Hapkido class the other day so I decided to use him for practice and I got a weird vibe when I switched to ты on him.
I would say stick to вы and use only ты when you do know the person you're talking to. Same way with all the languages that have this formal/informal thing.
You could easily get an answer which would be like "excuse me but I don't recall breeding pigs with you".