"I would like a sandwich" was 'corrected' to "I want one sandwich". What is the register of "querer", please? Is it strictly "I want - gimme that thing!" Or can it be used in a polite sense as well? (I asked the same question about "Yo quiero una camisa verde", but haven't got any answer. Hope I'll be luckier here :-)
There are more polite ways to say "querer", but they mostly use the subjunctive mood (which only appears near the end of the Duolingo lesson tree), and they aren't as common in casual speech.
"I would like a sandwich" translates literally to "Me gustaría un sándwich", which actually might sound preposterous unless someone explicitly is offering you something.
A very polite and humble way to ask for a sandwich is "Quisiera que me dieras un sándwich" (it has no direct literal translation, something close is "I wish you gave me a sandwich", Google translate gives "I would like you to give me a sandwich", but it is actually quite different in tone to "Me gustaría un sándwich"), though you wouldn't use that phrase unless it is a matter of life or death (for example, if your child is starving or something like that); the connotation is that you don't you think you deserve the sandwich, and you don't believe the other will actually give you one.
Thank you DroppedBass (and apologies that I haven't come around to your detailed answer until now)!
I take it, when I go to a food stall, I can say "Yo quiero un sándwich" without being impolite or bad-mannered, whereas I might be considered a bit kooky for saying "Me gustaría un sándwich". Is that right? Based on https://www.spanishdict.com/answers/282236/ordering-in-a-restaurant-me-gustara-o-quisiera-o-quiero, it might be a local thing as well.
"Would you like a sandwich?" gets you two possible replies in standard English: "Yes, please" or "No, thank you". Not "Yes, thank you". Though if someone just plopped a sandwich in front of you, at the same time saying "Like a sandwich?" I can imagine saying, in surprise, "Yes" (hesitation) "- thank you!" But by that time you already have the sandwich, so "thank you" is appropriate.