Is it not possible to use "Uma xícara" here? Is there a real difference between copo and xícara?
Yes. Xícara = cup (usually for tea, coffee, or a measure for recipes). So it sounds unnatural to ask for "xícara de água"
So xícara is more like mug (more likely to be made of porcelain) for hot drinks and copo is more like glass for cold drinks?
Yes, xícaras are usually made of porcelain and copos of plastic or glass.... but there isnt a rule on hot or cold. But the main idea is that ;)
Qual é a diferença entre "Se faz favor" e "faz favor". Eu acho que não ha nenhuma...
No Brasil usa-se mais "por favor", em Portugal preferem dizer "se faz favor".
Why is wrong: "Um copo com água, por favor."
Because it is not usual! But to speak is de same thing!
It would be "A cup WITH water, please."
Keep mixing It up with Spanish "copa"
A minute ago there was a question using com instead of de and I read a comment that someone said if you say "copo de água" they'll respond that they only have "de vidrio"...is it the same question or am I confused?
Do you mean this discussion?: https://www.duolingo.com/comment/666928
It's just a jokey response and both "com" and "de" can be used, in fact, according to one of Paulenrique's comments in that thread, "de" is more common.
Why is "uma taça de agua por favor" wrong?