Who or Whom?
in Formal Language we use WHOM instead of WHO / THAT:
- As the OBJECT of the Relative Clause:
That's the girl whom I met at the party.
- AFTER A PREPOSITION (That would never be used)
That's the girl about whom Paul is talking all the time.
In Informal Language we omit this Relative:
That's the girl (whom) I met at the party.
and the preposition is placed after the verb it modifies:
That's the girl (whom) Paul is talking about all the time.
In Non Defining Relative Clauses,
because we can NOT omit the Relative Pronoun,
the FORMAL Structure is common:
Peter, whom I met yesterday, is working abroad.
Ms. Wilkins, with whom you are working at present, used to work with me.