Improper use of "who" and "whom" is among the most common grammar mistakes that people make. Even famous rock stars get it wrong. Sorry, Mick Jagger, but it's not "Who do you love?" -- it's "Whom do you love?" Whether you are a teacher or just a grammar enthusiast, you can use a few simple tricks to help others understand how to properly use these commonly misused words.
Explain the differences between an object and a subject. Understanding these critical parts of sentences is critical to understanding proper usage of who and whom. "Who" refers to the subject of a sentence, or the person performing the action. If you are asking "Who ate the cookies?" you are asking for the identity of the person responsible for committing the action. "Whom" refers to the object of a sentence, or the person subject to the action. If you ask "To whom did you give the cookies?" you are asking for the identity of the action's recipient.
Use a substitution test. Concepts like subject and object can be hard for some to understand. It can be easier to explain how to use a substitution test to learn which word is correct. One common substitution test recommended is to use "he" or "him" in the sentence. For example, for the sentence "Who is the best baseball player of all time?" you can use the substitution test by asking if it's correct to say "He is the best baseball player of all time" or "Him is the best baseball player of all time." If the answer is "he" -- which in this case it is -- you should use "who." If the answer is "him," you should use "whom."
Reorganize the sentence to make the relationship more clear. Multiple clauses and subclauses can make the subject and object of a sentence confusing. Writer's Digest provides the example of the sentence "It was Carl who broke all the pencils in the house." You can make the information in the sentence more clear by breaking it into two clauses: It was Carl. He broke all the pencils in the house. Therefore, you can clearly see that Carl is the object, so "who" is the appropriate usage.