Because in English either a "the" or an "a" is required before "brother".
Similar to "I drive car" not being correct. It needs to be " I drive a/the car."
For family relationships, you can drop the articles to imply that it is your relative.
For example, I can say "Father studies" or "Mother studies". It sounds a bit old fashioned and formal, but the implication is that it is the speaker's mother/father.
Similarly, I can say "Brother studies" or "Sister studies" with the implication that it is my brother or sister. Again, it sounds formal and old fashioned, but it is grammatical.
This construction is used extensively in the Berenstain Bears books: https://www.putmeinthestory.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/t/h/the-berenstain-bears-pet-show-6.1561153748.jpg
I'm confused as well. Are definite articles mandatory regardless of context? Because as I understand it, in this context it could go either way in English. It is stilted to say "brother studies," but not only is it still grammatically correct, the course has accepted things like "Sister is in the city and Mother is at home."
Granted, it is in Beta still, so either way this appears to be a bug of some kind.
In English, "nouns" require articles while "proper nouns" (the name of a person, place, or thing) do not.
Could this also be translated as "The brother does study" or "The brother is studying"? Or is it strictly "studies"?
"The brother is studying" would work. It accepted it for me. It is also correct English.
But "The brother does study" would only be correct if you are doing something like a comparison as in "The sister is not studying but the brother does study. It is more of an emphasis that he is indeed studying.
I gave them the answer they want, but it is something I would never say. " The brother studies." No. Never. "Brother studies." would be a stretch, but more natural. In a regional way, a person would say, "Bubby is studying." or "Bro is studying." Basically, Brother is being uses as a name.