Learn French: Merci beaucoup, Monsieur. Je vous en prie. - Thank you very much, sir. You're welcome.

Literally, it translates to something like "I pray you of it" or "I beg you of it," but it is the standard, formal way to say "You're welcome" in French. Because the literal translation makes no sense in English, it is best to memorize this entire phrase as a single set expression.

Why do we use vous in Je vous en prie instead of te?

We use vous because the speaker is addressing a Monsieur (sir). Vous is the formal "you" used to show respect or polite distance. If you were saying "You're welcome" to a friend, family member, or child, you would use the informal "you" and say Je t'en prie.

Can I say Merci très instead of Merci beaucoup?

No, in French you must use beaucoup (a lot / much) with merci, never très (very). Even though we say "Thank you very much" in English, the French phrase is structured more like "Thanks a lot."

Do I need to add a person's last name after Monsieur?

No! In English, saying "Mr." without a name sounds strange, but in French, it is perfectly normal and very polite to use Monsieur completely on its own. It functions exactly like the English word "sir."