This is always a very confusing subject for many people. I know that it is a very advanced topic for primary school, but one of my friends asked me about it and I decided to write about it. To know whether you need to use who or whom, first you have to understand the difference between subjects and objects. You use
who when you are referring to the subject of a clause, and
whom when you are referring to the object of a clause.
For example:
Whom do I love? - The question is asking for the person that my love is directed.
Whom did you step on? - The question is asking for the person that was stepped on.
Who loves you? - The question is asking for the person that loves you, i.e. the subject of that sentence.
Who stepped on Nancy? - The question is asking for the person that caused the action.
You can obtain a worksheet from Georgia Southern University at the following link:
http://class.georgiasouthern.edu/writingc/handouts/whowhom1.htm
I know that it is a little bit confusing. Please let me know if you any questions or concerns.