Translation:Can I turn on the light?
There's no pronoun in the Chinese sentence. How do we determine whether it's "Can I" or "Can you" or "Can we / they" when translating it into English?
There is no i or you in the converse sentence why should it be" can the light be turned on?
Do you mean, why should it NOT be "can the light be turned on?" I think that's also possible as well as "can we turn on the light?"