Why is मेरी दादी चल रही है not accepted? Is the sentence really technically incorrect without the formal plural हैं ?
They are a mistake dadi is grandfather, grandmother is nani
Both दादी and नानी are 'grandmothers'. दादी is your father's mother and नानी is your mother's mother.
'Grandfather' would be either दादा (father's father) or नाना (mother's father).