Who/Whom wrote the letter?
Who
... dog is barking outside?
Whose
... cell phone keeps ringing?
Whose
The teacher to ... I spoke with was very kind.
whom
.... camera is this?
Whose
Who/Whom should I vote for?
Whom
.... is your favorite writer?
Who
Do you know ... is coming to class?
who
We wondered who/whom the book was about.
whom