One of my students asked me yesterday on Facebook to help her to rewrite a sentence using "who" or "whom". They are going to have a test, a test that is a hundred percent grammar; or close to hundred percent grammar.
Since "who" and "whom" are both pronouns and are both used to give more information about people, they are usually confusing for learners of English.
The rule says that "who" is used when we are referring to the subject of the clause and "whom" is used when we are referring to the object of a clause.
Our example is “Peter is talking to the man. I met him at the party.”
Our subject is "Peter" because he does the action and the object is "the man" because the action is done to him. The clause “I met him at the party” refers to the man. In this case, "whom" is the appropriate pronoun to be used.
There is another way to decide whether to use "who" or "whom". Form a question; if the answer is him them use "whom" and if the answer is "he" then use "who".
Example: Who is Peter talking to? Peter is talking to him. (him = whom; he = who)
I am not sure that this tip can help Vietnamese learners of English since the usage of "he" and "him" is also tricky for them.