"Sir, do you want milk?"
Translation:Señor, ¿quiere usted leche?
Duo has always been telling us that the difference between an assertion and a question is only intonation (and the question marks in writing). So, why "¿usted quiere leche?" is wrong??
"Do you want milk?" is certainly "Quieres leche?". Must we assume that the use of "senor" automatically infers formality necessitating the use of "usted"?
Yes, I believe that's what Dúo is trying to convey with their "sir/ma'am" sentences.
Senor always implies third person singular. Always. Never second person familiar. Always. My problem with the exercise is usted is unnecessary.
It's not quite accurate to say "Señor always implies third person singular"). Third-person speech occurs when "señor" is used as a noun meaning "man" or "gentleman" (e.g. "El señor quiere leche") but not when it's used as a form of address.
When you directly address a person, such as in the topic sentence here, you use a second-person form by definition. Both "tú quieres" and "usted quiere" use second-person pronouns and verb forms. It just so happens that in Spanish, the same verb conjugations are used for both formal second-person singular subjects (e.g. "usted") and third-person singular subjects (e.g. "ella").
You can certainly omit the "Usted" before "quiere leche" but remember that in spanish the use of pronouns along with the conjugated verb is more an emphasis than a redundance.
Because "vosotros" is second person plural and is generally used as informal, the formal way being "Ustedes". So, as you're addressing to a Sir you'll be polite and use the second person singular and formal "Usted".
Why is quiere put before usted? Would "Señor, ¿usted quiere leche?" also be correct?
Yes, it is correct too, and also more natural "¿quiere usted leche?" is a really polite way of saying the same.
Acceptable answers are: ¿Usted quiere leche? And ¿Quiere usted leche? And ¿Quiere leche usted? They seem to prefer, “¿Quiere usted leche?” Why? It seems to me that that is the most awkward.
"Usted" is the formal form of "tú" and when using it you always conjugate the verb according to the third person singular instead of second person singular. So as we're talking with a Sir you must be polite and use "usted" therefore conjugating "querer" as "quiere", "quieres" would be a conjugation for "Tú" which is informal.
Quieres leche is the informal way to say, "do you want milk.". It should be an acceptable answer.
When using the usted form, you must conjugate in the 3rd form (The same as Él and Ella)