I could sort of hear that this was interrogative rather than declarative. But Ukrainians I know would introduce the interrogative with "чи" -- why doesn't Duolingo?
As a native speaker of Ukrainian I almost never use чи unless I want to sound really formal/polite.
"Чи не бажаєте ви випити чашку кави?" - "Would you drink a cup of coffee?" sounds natural and formal.
"Чи вони грають у хокей?" - "Do they play hockey" - sounds okayish, but a bit weird if I speak to a friend. Might be regional though. I'm from Kyiv.
"Чи" does not have to be used in interrogative sentences. It can be used to emphasize the interrogative nature of the sentence. It is often used in the colloquial speech.
Just as you do not use чи to open an inquisitive sentence, the English equivalent does not require DO. If the inquisitive inflection is in the spoken or there is a ? at the end of the written sentence then "do" is implied"
In general would хокей be most likely to refer to ice hockey, or what the UK would just refer to as hockey, or field hockey? And what would be used to differentiate or indicate the other form?
"Are they playing hockey?" or "Do they play hockey?" Which one is correct for "Вони грають у гокей (хокей)? Or both...?
As I understand it, Ukrainian doesn't have the verb forms we have in English, so you would have to go by context and intonation. Sort of like the чи that started this thread: they don't use it because intonation and context make it clear that it's a question. But English speakers like to put it in because that's how our language works.
<rant>And гокей seems like a lost cause. Even though it is lifted from an English word, that starts with the Latin letter H there seems to be an insistence on turning it into Cyrillic Х despite the availability of the actual phonetic transliteration (Г). One could argue that "that's just what it is in Ukrainian" -- but then Ukrainians learn English and insist that NYC has a borough named "Manchattan!" </rant>
In Canada, Ukrainian-speakers say: "Чи вони грають Гокей?" &/or "Вони грають гокей?" (We NEVER use an 'y' in front of 'гокей/гокій' /'hockey'. It's not needed: One 'plays hockey' NOT 'one plays IN hockey'. A person 'PLAYS THE GAME' not 'Plays IN game.)
We use the letter 'Г' for 'hockey' (гокей/гокій) not a 'X' which makes the game into 'KHockey' not 'Hockey'. This spelling should be respected since the game of Hockey originated in Canada!
Depending what form of English you speak, hockey may or may not have originated in Canada. :) If you are in Britain, you would probably say that hockey originated in Europe, and ice hockey in Canada. Of course, in Canada you would say that field hockey originated in Europe, and hockey in Canada. Hence my earlier question about which version Ukrainians would usually mean and understand from хокей.