"It is not your father's lamp."
Translation:No es la lámpara de tu padre.
Agreed. I once had an instructor from Peru and she devoted half a class period to the verb "tutear" (as in, ¿Puedo tutearle usted? -- "May I use the 'tu' form with you?") and how outraged she got when people used the familiar form with her when she felt they hadn't earned it.
If you translate it that way, it has a slightly different meaning, namely "The lamp is not your father's" or even better "The lamp doesn't belong to your father." I don't know if you can see the difference here to the translation that they offered but I feel that there is one..