Hubiera v. hubieras

Question

Si ________ más atento con ella, María no se habría ido. If you had been more polite with her, María wouldn't have left.

I answered hubiera estado not knowing whether to use the tú form or usted form. Is there a reason using the usted form is incorrect?

Silvia Piriz · Accepted Answer

Hola Linda G.
Using usted here is not grammatically incorrect. Both forms are possible:

Si hubieras sido más atento con ella, María no se habría ido. (tú)

Si hubiera sido más atento con ella, María no se habría ido. (usted)

The difference is not grammatical, but contextual. In this exercise, the sentence is framed in an informal, personal context, so the expected form is tú (hubieras). That’s why hubiera (usted) is marked as incorrect in this specific case.
In real usage, either form is perfectly valid as long as the verb form matches the subject (tú vs. usted) consistently.
Saludos
Silvia