תשובה אחת
באופן כללי who's זה במקום who is
where-היכן
were- היו
who מתייחס לנושא במשפט-who wants to go home?
whom בהקשר "למי"- whom do you believe?

how can you tell when your pronoun is the object of a verb or preposition? try substituting he or she and him or her. if he or she fits, you should use who. if him or her fits, you should use whom. keep in mind that you may have to temporarily rearrange the sentence a bit while you test it.