How to use 'su' and 'tu'?
Hello,

I am confused on how and when to use 'su' and 'tu'. Can I use both? For example, I am your friend - Estoy su amiga or Estoy tu amiga.

Please explain.

Muchas gracias!

Question

How to use 'su' and 'tu'?
Hello,

I am confused on how and when to use 'su' and 'tu'.  Can I use both? For example, I am your friend - Estoy su amiga or Estoy tu amiga.

Please explain.

Muchas gracias!

Answer

Well ~actually «Su» can also mean "your" (singular) in a formal conversation, and "their".

● Tu casa → Your house [informal]
● Su casa → His/her/its house
● Su casa → Your house [formal]
● Su casa → Their house
 
you gave some examples,I will correct them for you:
I am your friend "Soy tu amiga" or "Soy su amiga".
that's te correct way :)
I hope I've helped you.

Answer

I forgot ...
You can also use the plural way like:
Tus amigos=you're friends(informal).
Sus amigos(Formal).