Learn Italian: Il biglietto è un euro o due euro? - Is the ticket one euro or two euros?

The Italian number one changes its spelling to match the noun it goes with, just like the English words for "a" or "an". Because euro is a masculine noun starting with a vowel, it takes un. You only use uno for masculine words starting with certain letters, like a z or an s followed by another consonant.

Why doesn't euro get an "i" at the end for the plural after due?

The word euro is an exception to the normal plural rules because it is invariable, meaning it never changes its ending. Whether you have one, two, or a thousand, it is always written and spoken as euro.

Why doesn't the sentence start with an equivalent of "Is"?

In Italian, you do not need to change the word order to turn a statement into a question. Il biglietto è can mean "The ticket is" or "Is the ticket?". You simply raise the pitch of your voice at the end of the sentence when speaking to show you are asking a question.

Why does è have an accent mark over it?

The accent mark shows that this is the verb meaning "is". If you remove the accent mark, the word e changes its meaning completely and becomes the word for "and".