"They stay with their maternal grandfather." should be an accepted answer.
I think its not accepted because the word Hindi word for maternal is not there.
No, it is. नाना means maternal grandfather, while दादा means paternal grandfather
No the Hindi word for maternal is मीतृ and that word is not in the sentence. Yes नाना is the word for "(maternal) grandfather" but the word "maternal" itself isn't there. If someone said ये मरे नाना हैं। And an English speaker asked what the person said the best translation would be "This is my Grandfather" because we don't make distinctions between paternal and maternal in English. When some one uses तुम you would translate it as "you" not "you plural" the word plural isn't there but the information is understood in the same way the maternal is not there the information is just understood. And since the actual word for maternal is not there, I am saying that is probably why it doesn't accept that answer, not that that answer would be entirely wrong.
There is no one correct translation. It depends on what is being said. Sometimes we simply translate it to "grandfather", sometimes the fact that it is the maternal grandfather matters. You decide how dynamic or formal your translation should be on a case-by-case basis. Since this sentence lacks context, they should allow us to be as exact as we want. Maybe somebody is answering that way because they're confused which is which and wants to check with the program. Why prevent a language learner from using a language learning software to learn a language? ^,^
Why does naanaa not change to naane? I am thinking it is in the oblique case, because it is followed by the postposition ke saath.
This is a good question. Perhaps it is an exception to the rule? Maybe a Native speaker can help here?
नाना becomes नानाओं in the plural. I'm not entirely sure why, though, sorry. I'm hoping someone with a bit more experience with the grammar could clear that up.
I just bought an italian hindi grammar book and there was a part about exceptions for the oblique flexion of nouns: there's written that some tatsam nouns (the ones that come directly from sanskrit), all the relatives' nouns (father, grandfather, uncle ecc.) and the agent nouns that end in "ता" (search on the net if you don't know what they are) are not declined in the plural form and in the singular oblique case, but only in the plural oblique case (adding ओं): f.e. पिता (father): singular पिता, plural पिता, oblique singular पिता, oblique plural पिताओं.
P.S. and of course all of the people names and surnames and the city names are never declined. I don't know if it's the same for rivers' names or mountains' names.
Thanks so much, Jacky101000! That was exactly the answer I was looking for!