pronouns - "Who(m) will it be?" vs. "Will it be he/him?"

The accepted (and highly upvoted) answer to the question in the question What’s the rule for using “who” and “whom” correctly? states that the easiest way to find out whether to use who or whom is to try with he/him and see which fits.

But that doesn’t seem to fit very well in this case:

1. Who will it be?




2. Whom will it be?

If I replace with he/him there, it becomes:

3. Will it be he?




4. Will it be him?

– and I don’t know which of these is right, either. Is it really true that you can always associate who with he and whom with him? Or does that not always work? And which out of 3 and 4 is correct?

Answer

Check these out: 'Who' vs 'whom': 1, 'Who' vs 'whom': 2

It is usually "Who will it be?" for the reasons given in the second article.

For your second question, I think "Will it be him?" is better. These definitions may be helpful:

he

pronoun used to refer to a man, boy, or male animal previously
mentioned or easily identified.

"Everyone liked my father—he was the perfect gentleman"

him

pronoun used as the object of a verb or preposition to refer to a male
person or animal previously mentioned or easily identified.

"His wife survived him"