I dont know it but i searched it up on yt and it came out try checking there
Answer: When addressing someone who you are unfamiliar with, someone with higher authority, or someone who is older like a grandparent or principally, you use usted. When you are talking to someone familiar like your cousin or friend, you refer to them as tú.
Explanation:
Gustavo is just the adjective so that means that Rebeca is not there for correo porque
