"Grandfather eats apples."

Translation: दादा सेब खाते हैं।

[SrijanH]

why it is not 'khata' instead of 'khate' ?

[GoWiP]

It's because of the respect (formality) for elders.
In formal situations and for respected people (including all elders), plural versions are used in every context. For example, with पिता (father): मेरे पिता भारत में हैं - My father is in India. It's the same here.

[ZelieZazou]

Why are almost all the "grandfather" and "grandmother" sentences translated with the words दादा and दादी, not नाना and नानी? I am surprised these latter words don't come up more often, we also need to practice them.

[zeebo7]

They are also accepted as translations, but you are right दादा/दादी seems to show up more often.

[IgorRusski]

And how do I know it's "apples" or "apple"? Shukriya whoever replies

[GoWiP]

Context. So in this phrase you do not know.

[Darius568602]

In a previous sentence featuring grandmother eating, I was marked wrong for using the respectful form. So here I used खाता and I was marked wrong again.