Eres vs Es
I've been told that when describing someone, you use "es".
Ex: "Ella es desordenada" or "Usted es desordenada"
But when talking about the person you're with, like a friend, you would say "eres" instead of "es".
Ex: "Tu eres desordenada"
instead of
"Tu es desordenada"
Por qué?
4 Answers
It's simply a matter of Spanish conjugation.
Tú eres
Usted es
Ella / Él es
It will come more naturally to you if you just accept it as you know it to be without comparing languages. As long as you compare, you will continue to think in English while attempting to speak in Spanish and this will make your acquisition of the language much more difficult than it needs to be. 
El aprendizaje del español no tiene que ser una tormenta.
Welcome tormenta
The verb needs to be conjugated for each person differently, unlike in English, even though the verb to be does have a slightly more sophisticated conjugation.
Look for the conjugation on our tab
If you didn't use the correct conjugation with the right person it would sound like us saying:
They is .....
in English 
When to use soy, eres, es, somos, sois, and son?
This answered my question in a way, it also made me remember french class and how they have a similar rule for it.
