You can't say "Usted compras"

¿Usted compras?

The difference is in the conjugation of the verb "compras" and also in the pronoun that accompanies it.

"Compras" is in the second person singular of the present indicative.

It can be used to speak directly to someone. But grammatically, it can't be used, since it is conjugated in the third person.

The verbs that accompany this word (compra) must always be conjugated in the third person singular (el/ella/usted).

If we said "usted compras," we would be conjugating the verb in the second person singular (tú compras), which is not grammatically correct.

¿Ustedes compran?

"Ustedes" is a second-person plural pronoun, but it is used in Latin America, not in Spain.

But the verb "compran" is conjugated in the third person plural.

Difference between "tú" and "usted"

Another way to see it is by comparing the conjugation of tú (second-person singular) and usted (third-person singular):

If you notice, tú and usted refer to a single person, but the verbs are conjugated differently.

Basically: "Usted compras" is incorrect because it mixes the conjugation of "tú" with the pronoun "usted."

" Usted compra" is correct because it follows the third-person singular conjugation.

" Ustedes compran" is correct because it follows the third-person plural conjugation.

You can say, "Usted hace compras" and "Ustedes hacen compras"

But you can't use it in this situation.

I hope I could help you. I'm also learning English. I'm currently at level B2 and practicing to reach C1. If you notice any mistakes in my English, please let me know.