I think that taking a purist view will lead students to confusion when they come across the realities of native usage. I have a colleague in the States who told me that one of his colleagues teaches 'It is I' as the correct form. I understand the argument about copular verbs not taking objects, but does anyone really use it? Where there are alternatives, like if I was/if I were, there's always the formal cop-out- you can say that form x is preferred in formal language.
