Dịch - English-Spanish - Do you prefer Tú or Usted?

Morganno:

Tu traducción incurre en una paradoja, si el sentido de la pregunta radica en saber cómo prefiere ser tratada una persona, es imposible que te dirijas a ella tuteándola para averiguarlo.

I do not understand that..

It's paradoxical.
Because if you are speaking to someone, you are already using "tú" or "usted".
Here, for example, the person askes "prefieres", so this person is using "tú" anyhow.

Ok, I'll try in English. There is a contradiction in your sentence. When you ask someone in Spanish if they prefer "tú" or "usted", that implicitly means that you haven't been granted yet with the confidence to treat them informally. So, if you ask how they want to be treated, you ask it formally, you don't address them as "tú" but as "usted".