"You" in French - either plural or singular but then formal. Unless you address a kid or someone you're familiar with, use "vous". Canadians are less formal and more commonly use "tu" when addressing someone (whatever his/her age).
When hesitating (having a conversation with someone who's neither an elder or a kid, nor a member of the authorities), simply reply using the same form as the person talking to you.
Parlez-vous anglais? = Do you speak English?
Formal and rather litteral - more fluent forms would be:
Est-ce que vous parlez anglais?
Vous parlez anglais?
Unless addressing young children, wait until you're addressed with "tu" before using it. If addressing people you're not familiar with or any member of the authorities, make sure you use "vous" instead.
- Tu es stupide. (You're stupid)
- Stupide, toi-meme. (Speak for yourself)