who, whom

Who or who is a subjective pronoun ( a substitute for a noun, meaning a person,a place, a thing, or an idea--who always stands for a person ). It takes the place of the subject ( actor ) in a sentence--
Who ordered the pizza?
Who may mean "the person that" or "the one that"--
My neighbor, who works in the pizza shop, said they are having a special.
Who may introduce a phrase or clause describing a person--
Is that the guy who works for the pizzeria?

Whom or whom is an objective pronoun ( a substitute for a noun, meaning a person, a thing, a place, or an idea--whom always stands for a person ). It takes the place of the object ( someone acted upon ) in a sentence--
To whom should I give the bill for the pizza?
There is the guy to whom you should give the bill.
To whom it may concern...

Now that you know that, you can say--
"Whom should I tell that the guy who delivers the pizza is here?"


Note--You are probably familiar with subjective and objective when you use "he" and "she" ( subjective ), or "him" and "her" ( objective ). If your sentence rearranged, makes sense with "he", use "who". If the sentence, rearranged, makes sense with "him", use "whom"--
He ordered the pizza.
Who ordered the pizza?
My neighbor ( she works in the pizza shop ) said they are having a special.
That is the guy; he works for the pizzeria.
Is that the guy who works for the pizzeria?
I should give the bill for the pizza to him.
To whom should I give the bill for the pizza?
There is the guy; I should give the bill to him.
It may be of concern to him...
To whom it may concern...