I put "I want double". It was marked wrong, and I understand that this probably isn't a good answer, but the suggested answers are:
• I want the double.
• I want twice as much.
I grew up in the US, and I would never say "I want the double", nor have I heard anyone else here say that. Double in English is not a noun, so I don't think you can say this. But... maybe this is my ignorance though.. just wanted to check.
I think the other suggestion, "I want twice as much." is a much better translation.
In Canada when ordering a coffee at a coffee shop it's very common to ask for a "double-double", that is, coffee with double cream and double sugar. I have no idea if either word is a noun or adjective or adverb. But I wonder if I brought this Canadian-ism to Stockholm, how should I say it? En dubbel-dubbel, tack ? Then wait for a polite, Um, vad?
You are right of course that "I would like" is more polite than "I want", both in Swedish and in English.
But if the Swedish is 'Jag vill ha', then the correct translation is 'I want'. Only if the Swedish says 'Jag skulle vilja' would it be correct to translate as 'I would like'.
You must translate the words in front of you. If the text says "He is a dirty bastard", you must translate that as "He is a dirty bastard". You cannot change it to "I find him unpleasant" because the latter is more polite.