When do I know whether to use "tu" or "vous"?
By default it is always better "vous" which is the polite form for "you", which you would use for a teacher, while "tu" should only be used with friends and close family members.
DV
Related French A Level answers
All answers ▸Translate into French: My credit card was hacked last week, so I am reluctant to give out my bank information.
When do adjectives change to agree with the noun they modify in French?
How do I translate "the time when..." into French?
Décrivez un thème important traité dans le livre, le film ou la pièce que vous avez étudié(e)