A wise woman once said that both ‘who’ and ‘whom’ mean the same thing. When you choose one over the other, all you are doing is changing the word order and the formality or informality of the sentence.
Personally, I find ‘whom’ very nineteenth century and would like to throw it in the bin BUT if you are writing for an international examination (like IELTS), writing a thesis or an academic article, you might want to vary your sentence structures with a little bit of ‘whom.’
Here is the answer and an example of each one.
Use the he/him method to decide which word is correct.
he = who
him = whom
Examples: Who/Whom wrote the letter?
He wrote the letter. Therefore, who is correct.
For who/whom should I vote?
Should I vote for him? Therefore, whom is correct.
We all know who/whom pulled that prank.
This sentence contains two clauses: We all know and who/whom pulled that prank. We are interested in the second clause because it contains the who/whom. He pulled that prank. Therefore, who is correct. (Are you starting to sound like a hooting owl yet?)
We want to know on who/whom the prank was pulled.
This sentence contains two clauses: We want to know and the prank was pulled on who/whom. Again, we are interested in the second clause because it contains the who/whom. The prank was pulled on him. Therefore, whom is correct.
And there you have it. There is more to ‘who’ or ‘whom’ than that of course, but this little method should send you in the right direction.
Bye for now,
Grammar Girl

Thank you, Grammar Girl, for not letting “whom” die. It is a valuable word in the english language.
Another handy reference book, written in a fun-style similar to Grammar Girl’s book, is “Booher’s Rules of Business Grammar: 101 Fast and Easy Ways to Correct the Most Common Errors” (McGraw-Hill), by Dianna Booher. Take the free assessment at http://www.boohersrules.com to test your grammar skills.
Wow, that was the best tip I’ve ever seen for determining between who and whom. Thanks for that!