What are formal and informal pronoums and how to know which to choose?

A big difference between french and english are the way some pronoums are used. "You" can be translated in two different ways. You can translate it to "Tu" and "Vous". This is because "Vous" can be used to adressed more then one person. For example "Vous avez mangé les tomates." meaning "You (several people) have eaten the tomatoes." But it can also be used when talking to one person, another word for "Tu". This is the case for a more formal context, when you do not know the other person very well or have a more formal relationship with them, for example a teacher or a boss. It is considered a mark of respect. So "Vous avez mangé les tomates." can be refering to only person having eaten the tomates. You can only know which one it is based on the context. Generally, "Vous" is the default and "Tu" is for friends, family or if someone has told you can use the less formal "Tu". This is called tutoiement and vouvoiement. In school, for example, I would "vouvoie" my teachers and "tutoie" my fellow students.