When do you use 'tu' and 'vous'?

'Tu' and 'vous' are both subject pronouns, and are used to replace nouns in a sentence or phrase, when that person is doing the action. 

Tu = you (singular)

Vous = you (plural AND singular formal)

So in French, there are two different ways of saying 'you' (singular) - you can use either 'tu' or 'vous'. The first form, 'tu', is used when in informal settings, for example when talking to acquaintances, family or friends. The second form, 'vous', is used in formal settings and can be used to show respect, for example when speaking to strangers and elderly members of the community, or people of importance, such as your Headmaster, your boss or the Queen.

There is no set rule, it is a general feeling as to which form you should use. You have to use your gut instinct as to whether it is appropriate to 'tutoyer' (use 'tu' towards someone). What I mean by this is there is no real 'right' answer, only what is appropriate for the circumstances you are under! For example, you would use 'tu' when talking to your Grandfather, despite the fact that he is an elderly man (and therefore worthy of respect/politeness), simply because he is a relative! And despite your Headmaster/Headmistress being an acquaintance, my advice would be to use 'vous' since you are still trying to show that level of respect for him/her. Each situation is unique, so take the time to think about the level of politeness/respect you are trying to show, and how formal/informal the situation is.