It depends on who gives you the order. If the order is given from sb else like your boss then its must. But when it is your inner need or urge then have to. In poland we have just one word for that "musieć" so you have to choose wisely :D
That is not a distinction that is made in any of the dialects of English spoken in the areas of the United States in which I have lived. Quite honestly, it sounds like the sort of distinction one might only find in an English as a foreign language class.
"Must" should definitely be accepted for "musieć". Are you completely sure it isn't? Maybe you made some typo?
Both versions work, I have no idea which one is more popular nowadays. But staying with 'euro' as the default makes it easier to understand why it doesn't undergo any declension in Polish.
I think I didn't pluralize it when they were first issued, but now I do. I wonder what other English speakers do.
Very generally in the British Isles (and it varies), I've noticed that the Irish will say "twenty euro", and the British will say "twenty euros".
The Republic of Ireland has the euro, and the U.K. still has the Pound, so perhaps more daily familiarity with the currency turns it into "twenty euro"?
I'm totally guessing, like. :)