Do you have a euro?
I saw the above sentence and do not understand why you say "a" instead of "an"?
You say Do you have "an" umbrella?, but Do you have "a" pound?
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Because "U" is here a semi- vowel/ semi - consonant. The same way "w" is a semi- vowel. When they stand at the beginning of a word, they operate as virtual consonants, so the "an" - which is only used for words whose first sound is a vowel- doesn't work.
A few words to exemplify this: union, university, yahoo, yellowish, yell, winner, wimp, walk, yen.
You would use "a" and not "an" with these words.