Can someone explain when to use "who" and "whom"?

who: when you want to use it as the subject (to test it, replace it with "he")

Whom: when you want to use it as the object (to test it, replace it with "him")

Example 1:

To whom it may concern

in order to test this sentence, just cover up everything from the beginning of the sentence until "to whom" part so that you're left with "it may concern." Do you think it will be more appropriate to have him or he in this sentence?

It may concern (him).

because "him" will sound more appropriate in this sentence, then "whom" will be more appropriate.


Example 2:

This is the guy who hired me.

cover up everything from the beginning of the sentence until you reach "who", your sentence should look as follows:

(he) hired me.

because "he" will sound more appropriate in this sentence, then using "who" is correct.

hope this helps.