"Grandmother eats fish."
Translation:दादी मछली खाती हैं।
As a way of showing respect, you treat words referring to people older than you or in a position of authority as if they were plural. So, दादी is treated as plural and conjugated with हैं. This is not a grammatical requirement but something that is usually done in practice.
Yeah you are correct . I didn't want to type the whole thing that's why I wrote in short.