When do you use the word who or whom?

Use "who" when a personal pronoun in the same position would be in the nominative case. If you know syntax, then use "who" for subjects and predicate nominatives.
Example: "Who says so?"
"He says so?"
Likewise, use "whom" for the objective case. This is trickier, but stay with me. "Whom did you see at the game?"
Now, I can't say, "Him did you see...?" but I can say "Did you see him...?" Are you following this?
Or try these 2 sentences:
I knew someone who loved me. (She loved me.)
I knew someone whom I loved. (I loved her.)
As above, use "whom" for direct objects, indirect objects, or objects of prepositions.
Hope this helps. I can't give you a crash course in grammar in 1,000 characters, or 1,000 words.

Comments

..."For whom the bell tolls", and for whom the sentence is parsed.
For whom the vote is cast.
.
To whom is the package addressed.
.
Well put Professor Sandman.
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Ed
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by Macmedic and yet ... here I am on August 11th, 2008
Thanks, Ed.
Even though I'm retired for nearly three years now, I can't seem to quit teaching.
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by Sandman on August 11th, 2008
Professor Sandman, et al.

In practical terms we each have three (3) occupations,
student, later teacher, and lifelong salesperson.

We may do other things, however, we have these three occupations in universal commonality.

Ed
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by Macmedic and yet ... here I am on August 29th, 2008
Thanks for this, I always get it confused as well.
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by Anonymous on September 11th, 2009
Top paraphrase the Ragpicker in "The madwoman of Chaillot," I'm a language pimp.
My daughter tells me, "You're retired. Quit teaching."
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by Sandman on September 11th, 2009
Hey Professor Sandman,
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Do you agree that the term "practice" as used in law and medicine is derived untimely from a Sanskrit word?
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Ed
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by Macmedic and yet ... here I am on September 11th, 2009