Answer: tu applies to the one person you are talking to
Explanation:
so let’s say you’re talking to a friend you would use tu but if it’s someone with more respect or a more formal situation you would use usted
Answer:
Your best friend
Your teammate
Your sister
Explanation:
You shouldn’t address your elderly neighbor with tú because it’s disrespectful. Using the form tú is with someone you already know and the form ustéd is for people older than you.
Hope this helps :)