How do you know when to use ''tu'' or ''vous'' in French?

In French, the use of tu or vous is determined by the level of familiarity or formality between the speaker and the listener.

Tu is the informal singular form and is used when talking to someone with whom you have a close relationship, such as a family member or a close friend.

Vous is the formal singular form and is used in more formal situations or when addressing someone you do not know well, such as a stranger, a customer, or a teacher.

Additionally, vous can also be used in the plural form, regardless of familiarity or formality. This is used when addressing more than one person, regardless of the relationship or formality of the situation.

It's important to note that in some regions of France and in some situations, such as with younger generations, tu may be used more frequently, even in more formal situations. However, as a general rule, it's best to default to using vous until given permission to use tu.