Constructions where nominative and accusative are in alternation
There are a number of constructions where both cases are found. In most, the nominative is restricted to formal (or very formal) style, with the accusative appearing elsewhere.
(a) Subjective predicative complement
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i.a. It is I who love you.
i.b. It's me who loves you.
ii.a. It is I she loves.
ii.b. It's me she loves.
iii.a. Yes, it is she!
iii.b. Yes, it's her!
iv.a. This is he / These are they.
iv.b. This is him / These are them.
v.a. ?The only one who objected was I.
v.b. The only one who objected was me.
vi.a. *This one here is I at the age of 12.
vi.b. This one here is me at the age of 12.
Probably the most frequent use of a nominative case predicative is in the it-cleft construction, as in (i-ii). And here we can make a distinction according to whether the pronoun would be in nominative or accusative case in the non-cleft counterpart: compare (i) with I love you and (ii) with She loves me. In the former, the accusative version (i.b) certainly has an informal flavour, whereas in the latter the nominative version (ii.a) seems very formal and the accusative (ii.b) relatively neutral in style. Nominatives are also found with it + be without a following relative clause, as in (iii.a). This is considered very formal -- and in response to the question Who's there? the nominative version It is I would be widely perceived as pedantic. The other main construction where a nominative is quite commonly found, again in formal style, is with a demonstrative as subject, as in (iv). It might also be used in if I were he (but hardly ?if you were I). Elsewhere, it is again likely to be perceived as somewhat pedantic. Most speakers would avoid examples like (v.a) -- which is easily done by reversing the order (I was the only one who objected). The context for (vi) is one where we are looking at an old photograph; it is difficult to imagine that anyone would use a nominative in construction with the following PP that we have here.