Interrogative pronouns are words that have been given two jobs. First, they are used to ask questions. Second, interrogative pronouns also function as stand-ins for nouns or noun phrases. There are five interrogative pronouns in English: who, whom, whose, what, and which.
In most cases, a pronoun refers to an object or person that has already been named. That person or thing is called the antecedent. It is interesting to note that, although the words who, whom, whose, what, and which are, indeed, pronouns, they refer not to a known antecedent but to an unknown one for the simple reason that the question is being asked to determine who or what that antecedent is.
For example, in the question “What do you want for dinner?” the interrogative pronoun is being used to ask for information regarding what isn’t known — the respondent’s dinner preference. The antecedent is assumed rather than stated; it is whatever food choice the respondent makes. Perhaps the answer is “pizza,” in which case that clarifies the antecedent.
The question “Who was at the door?” contains the interrogative who. The speaker knows something nonspecific — that someone was at the door. It is only when the question is answered that the antecedent is supplied. The answer can be very specific, such as the individual’s name, a little less specific, such as a category like salesperson, or as nonspecific as the question, such as no one.
Whose is the only interrogative pronoun that is also possessive. It asks about a known object that belongs to an unknown antecedent. “Whose dog is that?” recognizes that there is a dog that belongs to someone; it is only when that person is named that the antecedent becomes clear.
The interrogative pronouns who, whom, whose, what, and which are never found in a linguistic stream other than a question. This is not to say that these words aren’t used in other contexts — they certainly are — but they are given a different grammatical function in such cases. They become relative pronouns used to illuminate an adjective clause. For example, in the statement “A doctor is someone who heals the sick,” who introduces the descriptive phrase “heals the sick” and is therefore not an interrogative pronoun.
People often confuse interrogative pronouns with question words. Youngsters learn in elementary school that they should use the words who, what, when, where, why, and how to form questions. Of these, only who and what are also interrogative pronouns. The other words are used not to determine an unknown that is a specific noun but rather an unknown that names time, location, reason, or manner.