EXCERPT FROM THE CAMBRIDGE GRAMAR OF THE ENGLISH LANGUAGE:

We look at the contrast between nominative and accusative cases. [Nominative =he/she; Accusative=him/her] Here we find a considerable amount of variation and instability in the [English]system. There are a number of constructions where the nominative is asociated with formal style and the accusative being strongly preferred in informal speech and writing. Because of the tendency of older prescriptive grammar texts to accept only formative style as "grammatically correct", there has been a tradition of criticising the accusative alternants, and the stigmatism attaching to such accusatives has given rise to a certain amount of hypercorrection, with nominatives being used in constructions where the traditional rules call for an accusative. There is ONLY one function where the nominative case appears to the exclusion of the accusative, irrespective of style level: as the subject of a finite clause. Compare:

I made up some new curtains(correct) versus Me made up some new curtains (Incorrect)
I think he is mad (correct) versus I think him is mad (Incorrect)


Constructions where both the nominative and accusative forms are in alternation:

Yes it is she. (correct nominative form) versus Yes it is her (correct accusative form)
This is he. These are they. (correct nominative form) versus This is him. These are them. (correct accusative form)
It is I who loves you (correct nominative form) versus It is me who loves you (correct accusative form)
The only one who objected was I. (correct nominative form) versus The only one who objected was me. (correct accusative form)
This one here is I at the age of 12. (Incorrect nominative form) versus This one here is me at the age of 12. (correct accusative form)



The nominative forms are considered very formal-and in response to the question, "Who's there?" the nominative version "It is I" would be widely perceived as pedantic compared with "It is me."
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