Can you please elaborate more on why that is the case. It doesn't seem natural to me, but then again, I might not be accustomed to the "very formal" approach.
Me too. The copulative requires the nominative case. Very formally, some texts insist on 'It is I', though we all say 'it's me'. Similarly, if I were writing a business letter or an academic paper, I would write, 'if you had been I, you would have shot the proctor too.' In theory. On second thought, I take it back-- I don't think I'd ever use it.
To know if anything is 'correct', what options do we have? (1) check a grammar book, (2) google or other statistical research, or (3) ask someone. I myself tend to walk the line between prescriptivism and descriptivism, relying more on either (1) or (2) to support whichever viewpoint suits my feeling on the matter.
In this case, (1) the grammar text offers me only marginal-- if any-- s