Grammer Corner: Who & Whom

I have recently been struggling with this concept. When should I use who and when should I use whom. If mixed up an English instructor will be very unhappy. Who says ignorance is not bliss! I have successfully avoided learning the difference between the two for quite some time.
The rule states: ""Who" and Whoever" are subject pronouns; "whom" and "whomever" are in the objective case."http://web.ku.edu/~edit/whom.html
Now that I know the rules hopefully I will be able to distinguish between who and whom.