You would do well to use the links we've prepared for the course.
Read the Tips & notes, on the first page of each lesson you’ll see an TIPS. Click on that.
Here is the Link for the ABC Skill https://www.duolingo.com/skill/el/ABC/tips-and-notes
Read the drop down hints. Pass your cursor over a word and a list of words will appear. The top word/phrase is usually the best choice.
For this translation you would have seen: ypsilon so there would be no problem with the U which is a false friend...looks similar but isn't.
Use the Report options at the foot of the exercise page to Report issues such as **My answer should be accepted.” etc
Here is the Greek Forum with other Links
So, in English it's upsilon, in Spanish it's ípsilon, in Bulgarian it's "epsilon"(eh-psilon) and in the more modern-Greek-friendly English translation it's ypsilon. Apparently ύψιλον used to be pronounced like üpsilon would be in German, which is between 'u' and 'i'. That still doesn't explain why Bulgarian has 'e' though... maybe there was such a Greek dialect?
No difference at all. Here, the whole alphabet with many examples
Κάθε γράμμα είναι ένας ήχος! Η Αλφαβήτα με φωνήματα:/ Every letter is one sound (= the first sound in its Greek name)
You also get different ways to write the letters. I like the one with many tails and ornates.