Grammar Rules regarding the pronoun 'it'

"I want to be a doctor. I like helping people that's why I want to be it."
"Where's your new iphone? Can you show me it?"
What grammar rules govern the use of the pronoun 'it' in these sentences. They don't sound natural but I'm not sure if they are incorrect.

Answer

In the first example, the antecedent is "a doctor". The usual way to refer back to an indefinite antecedent (e.g., a noun preceded by the indefinite article) is one:

I want to be a doctor. I like helping people that's why I want to be
one.

As Swan in Practical English Usage (p369) states in his entry on one:

We often use one instead of repeating a singular countable noun.

• Can you len me a pen? ~ Sorry, I don't have one.

In the second example, there is a definite, identifiable antecedent. The usual pronoun for definite, non-human antecedents is it. So there is nothing wrong with it itself in Can you show me it? Some native speakers, however, may object to the order of the pronouns, and prefer: Can you show it to me?

There's a lengthy discussion on this latter issue, here:

Is it incorrect to say, 'Give me it'?

and here:

Direct object "it" in final position... grammatical?