Is there an easier way to know when to use WHO or WHOM?

Answer 1

"Whom" just makes you sound smarter.

If you want to sound smart, then use it.

Simple as that.

Answer 2

Remember your prepositions? UNDER the table, OVER the top, BESIDE the point...

By, for, from, in, to, with are common prepositions.

These words describe the relationship of an object to a verb or adjective.

If who/whom is part of a preposition, use whom.
If who/whom is not part of a preposition, use who.

(To whom) it may concern,

It concerns (who)?

I report (to whom)?

(Who) issued this report?

Answer 3

WHO is the subject form, like He/They.
WHOM is the object form, like Him/Them.

For "Who" substitute "He" or "They" -- these are clearly
used for the "subject" of the sentence or the clause, the
person "initiating" the action of the verb
For "Whom" substitute "him" or "them" - these are clearly
the "object" of the clause, commonly used with prepositions
or "receiving" the action of the verb

If you substitute using he/they
or him/them, does the sentence sound right?

If not try the other case.

Answer 4

Generally, who is a pronoun and whom is the object of a preposition.

Answer 5

if you can replace a word with "he" or "she," then it is the subject of the sentence and you should use "who." If you can replace the word with "him" or "her," it is the object and you should use "whom."