About (who/whom) were you speaking?Whom is correct because it is the object of the preposition about.
Distinguish between that and which—a frequent error—according to the kind of
clause you use, one that is critical and necessary to the sentence or one that is not.
Which clauses require commas to show that this part of the sentence could be left
out:
Her dog, which barks every morning at 5 A.M., should live on a farm.In this sentence, the clause, which barks every morning, is not absolutely critical to
the meaning of the sentence. The main part of the sentence could stand on its
own:
Her dog should live on a farm.The pronoun which is called parenthetical: as you can see, it is incidental and can
be left out of the sentence without affecting the meaning. Other examples:
The coat, which is Robert’s, was found in the theater.The coat—whether or not it belongs to Robert—was found in the theater.The house, which my brother designed, will be ready for spring occupancy.The house—whether or not my brother designed it—will be ready for spring
occupancy.CHAPTER 3 Verbs, Prepositions, and Pronouns^69
COMMON ERRORSNever use which to refer to people. For example:Incorrect: The person which is in charge is my least favorite woman in the group.
Corrrect: The person who (or that) is in charge is my least favorite woman in the group.