There may be a rule, but what is its purpose? What clarity is gained (or ambiguity avoided) by preferring "This is she" to "This is her"? The usage has degraded for a reason—because there was never really any meaning behind the distinction and it's never caused anyone a problem.
“This is she” vs. “This is her”
There may be a rule, but what is its purpose? What clarity is gained (or ambiguity avoided) by preferring "This is she" to "This is her"? The usage has degraded for a reason—because there was never really any meaning behind the distinction and it's never caused anyone a problem.