"It is a very small stadium."
Translation:Це дуже маленький стадіон.
"Це Є дуже малий стадіон." IS Correct as well. It is grammatically correct. My parents spoke this way as well. WHY do you mark the "є" in the sentence as "incorrect" when it IS a correct figure of Ukrainian speach used to date? It is unfair. Please correct this. "Це є..." = "It is...".
The copula "є" is used in set constructs, like "у мене є..." (I have a ...), in scientific, encyclopedic texts, for example, "Температура в 20 кельвін є необхідною для..." (The temperature of 20 kelvin is necessary for...), or for extra emphasis and formality. (forum.slovnyk.ua)
"Although there are Present, Past and Future forms of Ukrainian verb to be - бути, it is rarely used in Present tense. English words is, am, are don't have Ukrainian counterparts. Thus, the English four-word sentence I am a student is just two words in Ukrainian: Я студент." (Tips and Nots)
In this course it's not accepted because
1) It is not natural in standard Ukrainian
Using "є" is obsolete in modern Ukrainian and sounds unnatural. It is pretty common in some regions of Ukraine though, but not in the "textbook" Ukrainian.
2) Copying the structure of the English sentence
The idea is, if a learner actually does not speak any Ukrainian, they might write "є" simply because they are copying the structure of the English sentence. The developers of this course think that the users should learn that it's unnatural, even though it's technically not wrong grammatically. And if the users do actually speak some Ukrainian with the variation/dialect where "є" is natural (e.g. some Western Ukraine regions, Canadian born Ukrainians), they can understand that this course is also for people who don't speak Ukrainian at all and just go along with it, speaking the way they want at home, and following the "textbook rules" here :)