'Tu' and 'vous' and staying consistent
English has just one word for addressing someone, making it simple to talk to anyone. French splits the concept of "you" into two paths. You use the informal tu for a friend, a child, or a family member. You use the formal or plural vous for a stranger, a boss, or any group of people. Choosing the right form of address is your first step when speaking French.
The real challenge for English speakers is staying consistent. Once you pick tu or vous for someone, every verb ending, possessive word, and polite phrase in your sentence must match that choice. You cannot mix them when talking to the same person.
When you talk to someone using tu, everything they own also takes informal possessive words. You will use words like ton or ta for "your", and le tien or à toi for "yours".
If you are addressing someone formally, or talking to multiple people, you must use vous. That means their possessions take the formal or plural words votre, le vôtre, and à vous.
The choice between tu and vous even changes the everyday expressions you use to say please, excuse me, bless you, or cheers. French speakers call using the informal forms tutoyer and using the formal forms vouvoyer.
If you step on a friend's toe, you might say excuse-moi. If you step on a stranger's toe, you need excusez-moi. The same logic applies to asking for favors. You will use s'il te plaît with friends, but s'il vous plaît with a waiter or a group.
This consistency extends to almost all social interactions. If someone sneezes, say à tes souhaits for a friend and à vos souhaits for a colleague. When clinking glasses, use à ta santé informally and à votre santé formally. To say "you are welcome", use je t'en prie or je vous en prie.
The rule of consistency applies to a single addressee. You must never mix tu and vous when speaking to the exact same person. However, your sentences can legitimately contain both forms if you are addressing different people in the same breath.
When speaking French, you must pick your formal or informal style and stick to it. Every pronoun, possessive word, and fixed expression addressed to a specific person must match your initial choice of tu or vous.