eŭro

Two things to get right, and they are both about the letter E.

The currency is a common noun and takes a small letter: eŭro, exactly like dolaro, franko and pundo. The continent is a proper noun and keeps its capital: Eŭropo. So mi havas dek eŭrojn but mi loĝas en Eŭropo, and the adjective eŭropa (European) is lower case again because Esperanto does not capitalise adjectives of nationality the way English does.

The second thing is the eŭ itself. It is a diphthong, one syllable, not two: EŬ-ro has two syllables, not three, and the ŭ is the w of "how" rather than a vowel in its own right. In the x-system used for typing without the accented letters it is written euxro, which is the slug this page lives at; the proper spelling always carries the breve.

Being clipped from Eŭropo is the reason the word has no plural oddity and declines like any other noun: eŭro, eŭroj, eŭrojn. English wobbles between "ten euro" and "ten euros" depending on which official style guide has got hold of the writer. Esperanto does not wobble.