Who / Whom / Whose
Todos se usan para personas.
Who
Who se usa como sujeto u objeto:
That's the man who stole my car.
That's the man who I saw stealing my car.
Whom
Whom se usa sólo como objeto pero cada vez menos:
That's the man whom I saw stealing my car.
Sin embargo después de una preposición es la única palabra correcta:
That's the boy to whom I wrote a letter.
Whose
Whose (cuyo) se usa para el caso posesivo:
That's the lady whose husband is an actor.
That
That se usa tanto para personas como para cosas:
I think you're the man that I talked to on the phone.
Where
That's the house where I used to live.
When
That was the day when I met my husband.
Why
That was the reason why we left.