When should you use Vous or Tu ? This is one of the most subtle and complicated points in the French language. I think you could say that it has more to do with the culture than with the language itself.
Until you have a good feel for it, you can use the following rules of thumb:
If you are young (say, 24 and younger), use tu with anyone your age and younger, unless the situation is very impersonal (e.g. with a cashier in a shop).
If you are older than this, use tu with young children (in any situation) and with adolescents and young adults in social situations.
Use tu on the Internet unless you’re trying to be very formal, or you’re talking with someone whom you know offlilne and address as vous.
If you are in school or university, use vous with your teachers, no matter their age.
If someone who is your social equal (co-worker with the same job, for example) uses tu, you can safely use tu as well. This may not apply to social superiors.
In all other situations, use vous unless the person you’re talking to specifically asks you to use tu.
(May seem obvious, but needs to be included): Always use vous when addressing more than one person. This is a grammatical rule and you will not be understood if you use tu.
Again, these rules are very approximate and everyone will have a different opinion on this, but they should keep you from shocking anyone too badly until you develop your own style.
One last thing regarding the use of Tu or Vous : remember that if your non-Frenchness is obvious from your accent, French people will normally forgive impertinent uses of tu, so don’t stress out too much over this.
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