"I will come the next day."

Translation: मैं अगले दिन आऊँगी ।

[richard547513]

Why is अगले plural?

[vinay92]

It's not plural. It is in the oblique case. There is an implicit postposition को whose object is 'अगले दिन'. Think of this as being similar to the implied 'on' in the sentence, 'I will come (on) the next day.'

[paul376479]

Is there a clear grammatical reason (other than it just 'is') why 'Agla din mei aiungi' is not also correct?

[Wilbur111]

Because it's an SOV language. Meaning the sentence starts with the Subject, then has an Object, and then the Verb is at the end.

You will perhaps notice that the time element tends to come right after the Subject. I don't know if that's a rule or not but I treat it as one and it's kept me in good stead.

[Farnoosh.Frh]

Why not say अगला?

[Gita-ji]

Because there is an implied post-position, को, after दिन whose adjective, अगला, will now have to be in oblique case.
So अगले।

[Kae486202]

Ditto

[AlexRau9]

Is "मैं अगले दिन को आऊँगा" correct, or does it sound weird from a native viewpoint?

[vinay92]

It sounds a little weird (but not completely wrong) to use को with आना and जाना. It is only explicitly used in certain contexts.

[paul376479]

No it does not sound weird... it is exactly right from a native viewpoint :) ... Only weird within English grammatical order