French has two "you" forms — tu (informal) and vous (formal/plural). Picking wrong is the #1 mistake learners make. Locals don't always switch explicitly — but the contexts are clearer than they look.
French doesn't forgive the wrong choice the way English forgives "you." Tu-to-vous error is a social signal: too familiar (tu when vous is expected) is presumptuous; vous-to-tu is cold. The grammar is also different — verb conjugations change.
The non-obvious rule: vous is also plural. Talking to two people, always vous regardless of intimacy. Some French use "tu" for close colleagues within 6 months; others never. We can't read minds, but we can read contexts.
Either "tu" or "vous" with the reasoning. When it's a coin flip, we tell you how to break the tie (offer tu first, wait for them to switch to tu).
The exact phrases to use when you decide to switch from vous to tu: "On peut se tutoyer?" / "Tu peux me tutoyer."
Where the wrong form really damages — age gaps (older person to younger, always vous first), authority, religious contexts.
Yes. Vous is the "polite default" for any unknown. It signals respect. The downside: in casual contexts, vous makes you sound stiff or distant. Once someone invites tu ("tu peux me tutoyer"), switch.
Default to vous in business email. Friends and close colleagues: tu. If unsure, follow what they use first — mirror it.
Yes — family is always tu. In-law families and elderly grandparents also. Even children address parents as tu. (Older cousins/uncles may vous if they're very formal.)