"I am your grandfather."
Translation:मैं तुम्हारा नाना हूँ।
तेरा is what you use when talking to very close friends only, otherwise it is quite disrespectful. तुम्हारा is for most other friends, colleagues, acquaintances, people you know. And आपका is the formal version, use this the first time you meet someone, if they are older than you, or in a position of respect like your boss or a religious figure.