Which is correct?
The winning high jumper was her/she
That may be she/her waiting at the bottom of the stairs
It was probably they/them swimming in the Browns' pool
Could it have been she/her on the phone?
What I have been taught is that when the verb "to be" is present then one must use the subjective pronoun
So then "the winning high jumper was she"
because she is the subjective pronoun
Can anyone confirm that I am doing this correctly?
No one talks like that.
The first sentence, I would use "her." Actually, I would just say "She was the winning high jumper." No need to use the passive.
Her/them/her for your other sentences.