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This is sort of related to the argument of "This is he" vs "This is him."

While most people will say 'who' where 'whom' is appropriate or 'him' where 'he' is appropriate, 'It is I' and 'I as...

I was thinking in the instance of nominative vs accusative. But if you want to speak broadly, if you would say Он in Russian, say he. If you would say его, ему, им, or нем, say him. Likewise with...

Think about it like this:

He = он
She = она

him = его
her = ее.