"Do you want a glass of water?"
Translation:¿Quiere usted un vaso de agua?
Duo teaches three ways to ask "Do you want a glass of water. (They should accept all three.)
¿Quieres un vaso de agua? (You informal)
¿Quiere usted un vaso de agua? (You formal)
¿Quieren ustedes un vaso de agua? (You plural)
The usted is in Duo's sentence because they chose to ask the question to someone older, a stranger, or someone in authority.
They really need to include some indication that this is formal. When presented with "Do you want a glass of water?" to translate, I answered, "Quieres un vaso de agua" which was not accepted. If they had proposed, "SIR, do you want a glass of water, I would have known to use "Quiere usted."
The Department of Education at Universidad Francisco Marroquín (Guatemala City) has a Web page about this. They say that the preposition "de" indicates a measure or quantity. In other words, "un vaso de agua" means "a water measured by a glass. They also show other ways in which "de" could be interpreted. Note that number 3, "un vaso de mi prima" does not mean a glass made of my cousin, just as un vaso de agua does not mean a glass made of water.
1) vaso de cristal (materia prima); 2) vaso de cerveza (exclusividad); 3) vaso de mi prima (pertenencia); 4) vaso de juguete (función); 5) vaso de Tailandia (lugar de procedencia), and 6) vaso de adorno (función)
With "de" it might literally mean "a glass made of water", but it is they way that we usually say it! Maybe because it can also be "un vaso lleno de agua" which means "a glass full of water", where the word 'lleno' is omitted. I don't really know. Anyway, you can also say "un vaso con agua", they'll understand you, but is more common with 'de'.
So "querías" means "you would like", as opposed to "quieres" which means "you like", therefore the only difference between "¿Querías un vaso de agua?" and "¿Quieres un vaso de agua?" is "Would you like a glass of water?" and "Do you want a glass of water?" While your translation might be more polite or natural, it isn't what the question was asking for