Is "Tu kahaan ja rahi hai?" correct?

Why

Ishita noticed that both words mean addressing someone as 'you', which is true — but that alone doesn't capture the difference. In Hindi, 'तू' is the most intimate/informal pronoun — used with very close friends, small children, or when addressing God — but is considered rude toward elders, strangers, or respected relations like a grandmother. 'तुम' suits ordinary familiarity (friends, peers), and 'आप' shows respect — the three form a ladder of formality, not two options that are 'the same' and one that's 'different'. Test it: this is exactly why her mother got upset — saying 'तू' to a grandmother is as jarring as speaking casually to an elder stranger. Her mother is right: 'तू' is very intimate/informal and unsuitable for elders or respected people — 'आप' was called for with her grandmother.