who/whom

I’m frustrated that I still do not understand the difference between who vs whom clearly despite doing a lot of research on it. I know that who is for subject and whom is for the object.

My question is
is there a sentence like

blah blah blah, WHOM is a doctor? sounds very awkward and I can’t think of a situation that works.

I found the bill sale for my great-great-great grandmother Elisabeth, who is sold in 1850 in Cane River.

The sentence above always gets me! Because I thought that I was the subject, so Elisabeth is the object. Which is why I picked whom. Can someone explain this to me please?

Thanks

It sometimes helps to think of sentences with relative clauses as if they were separate sentences. (I am correcting the verb tense.)

I found the bill sale for my great-great-great grandmother Elisabeth. She was sold in 1850 in Cane River.

In the second sentence, “she” is the subject. So “she” must be in the subjective/nominative case. “Her” would clearly be incorrect.

When you combine the sentences by replacing “she” with a relative pronoun, the pronoun must also be in the subjective/nominative case. So “who” is required. “Whom” would be incorrect for the same reason “her” would have been incorrect.

“WHOM is a doctor” is always incorrect.

Ok thanks, that is what I needed. I wasn’t sure if there is a situation of WHOM is something. Now I know, it is so much easier for me. Thank you very much.

Who is the SUBJECTIVE case
Who is equivalent to: I, we, he, she

Whom is the OBJECTIVE case
Whom is equivalent to: me, us, him, her

Correct:
You went to the movie with HIM.
You went to the movie with WHOM?

Incorrect:
You went to the movie with HE.
You went to the movie with WHO?

As noted above, a quick way to remember the difference is that if you can answer the question with HIM, with an M, the word is WHOM, with an M.

Who did that? – He did.
You hit whom? – I hit him.

I have nothing else to add, but make sure that you really evaluate based on each clause. If you have a subordinate clause where the subject is either who/whom (or whoever/whomever), you need to make sure that you choose who.

For example: I want to hire whoever knows the area best.

Even though you’re hiring someone, the subordinate clause “____ knows the area best” overrides that. Even though that clause is the direct object of the verb “want,” you have a subordinate clause where whoever is the subject. If the subordinate clause acts as a DO/IO or even the object of a preposition, you may still use who if it’s the subject of that secondary clause/phrase.

I teach math, not English, but I agree with @bodangles.

That’s the easiest way for me to remember it, or to explain it to my own kids or the kids in my homeroom.

Turn the sentence around to make it a question.
If “he” is the answer, then it’s “who.”
If “him” is the answer, then it’s “whom.”

“Who left the water running?” He did, mom.
“To whom did you address the letter?” I addressed it to him.