You say tomato, I say tomatoe
1. In traditional, formal grammar, subject
pronouns are used after BE:
This is he.
Was it I?
In conversational grammar, object pronouns
are more common:
This is him.
Was it me?
2. Object pronouns are used for direct objects,
indirect objects, and objects of prepositions:
I saw him / her / it / them / you yesterday.
I gave the message to him / her / them / you (or
I gave him / her / them / you the message).
She bought a gift for him / them / you / me / us
(or She bought him / them / you / me / us a gift).
They were standing near (next to, beside,
behind, near, in front of, etc.) me / us / you /
him / her / it.
On a related point, those who continue to announce ?It is I? have traditional grammatical correctness on their side, but they are vastly outnumbered by those who proudly boast ?it?s me!? There?s not much that can be done about this now. Similarly, if a caller asks for Susan and Susan answers ?This is she,? her somewhat antiquated correctness is likely to startle the questioner into confusion.
I suppose you could call it an idiom, or more likely, snobbery.
English is a Germanic language, and in general Germanic languages take the nominative case for the object (sometimes called predicate) of the verb "be". Modern German does for all objects of "be", not just pronouns.
Old English did this too, but in the middle ages, English started to change under the influence of French and started using the accusative (me, her, him, us, them) after "be" instead of the nominative (I, she, he, we, they).
If it was true that modern English took the nominative after "be", we would say things like "That's they over there" or "The man who murdered Poirot is he!".
So if anyone tries to tell you that "This is she" is really their natural way of speaking ... they
a) have been dead for several hundred years
b) are a snob
c) have had this rule shoved down their throat by a snob