"The boy learns Korean, but he also learns English."
Translation:남자아이가 한국어를 배우는데 영어도 배워요.
Because 배우다 is an action verb and not a descriptive verb. Descriptive verbs (like 작다 and 좋다) use 은데 for verbs that have 받침 (a final consonant in a syllable block, like the ㅎ in 좋) and ㄴ데 for verbs that do not have any. Action verbs like 배우다 use 는데.
i know it's a bit redundant, but is there any reason for '남자아이는 한국어를 배워요. 그런데 그는 영어도 배워요' to be wrong?
It seems grammatically correct in my view, but the sentence asked by Duo does not contain a "full stop". It might be that.