"मैं अगले दिन आऊँगी ।"

Translation: I will come the next day.

[glarbish]

This exercise says that it should be अगले दिन whereas the exercise linked below requires अगला दिन:

https://www.duolingo.com/new-comment?fromLanguage=en&learningLanguage=hi&sentenceId=d507c62079cd4ade69e79a4fb5d63ac4

This seems inconsistent to me, but I may be misunderstanding. Can anyone explain if one is incorrect and, if not, what determines अगला vs अगले?

[RanzoG]

Think of this one as "ON the next day." There is an assumed pre/post-position (e.g. on, in, at , during) there in the deep structure of grammar even if it is going unsaid. The presence of this postposition puts the phrase "next-day" into the oblique case.