Interrogative pronouns are used in interrogative clauses, which may be either main clauses or dependent clauses (so called indirect questions). Most English interrogative pronouns start with wh-, and are therefore often called wh-words. Consequently, interrogative clauses introduced by wh-words are often referred to as wh-questions.
Many interrogatives can function either as determiners or as full noun phrases, i.e. as pronouns proper, while others can have only the latter function.
What colour is his house?
What did she buy for him?
Which side is better?
Which do you prefer?
Who did you meet today?
Whose car is this?
Whose is this car?
To whom did you send it?
In addition to the core cases of interrogative pronouns in this table, the class of wh-words includes the interrogative adverbs where, when, why, and how, as well as the interrogative subordinators whether and if.
What and which as determiners
Both what and which can be used as determiners in noun phrases with personal or non-personal reference.
The use of which is restricted to questions with a limited number of possible answers, which may be given either explicitly or by the context.
(1) Which alternative would you prefer?
(2) Which car is yours?
(3) Which singer did you vote for?
These questions all assume that the speaker and hearer can identify a limited number of alternatives among which the answer can be found. If no such pre-established set of answers is available, what is used instead.
(4) What alternatives do we have?
(5) What car did Roger Moore drive as “The Saint” on TV?
(6) What singer has sold the most albums in the history of pop?
Often, the nature of the expected answer makes it unlikely that a limited set of answers has been established in the discourse. This is true even if, in reality, there is a limited number of possible answers.
Thus, even if there is a limited number of days in the week (or the year), what is the normal choice in questions about times of the day, week-days, dates, etc., as illustrated below. Similarly, even though a book has a limited number of pages, without a pre-established set of pages under discussion, what is the normal choice of determiner.
(7) What day is the race?
(8) What time is it?
(9) What page did you get to?
Who, what and which as pronouns
As a pronoun, which is restricted in the same way as when it is used as a determiner. Thus, it is used only when the answer is to be found among a given limited set of alternatives. The choice of alternatives can be given in a postmodifying prepositional phrase, typically headed by of. Which can be used both about human and non-human referents and both in the singular and the plural.
(10) Which (of these alternatives) would you prefer?
(11) Which of the candidates would be best suited for the job?
(12) Which do you like the most, Bill or John?
If no set of possible answers has been established, who is used for human referents and what for non-human referents. In this case postmodification by an of-phrase is not possible (please remember that * in front of an example indicates unacceptability), although other postmodifiers occur (typically with the role of amplifying the wh-word).
(13) What are our alternatives?
(14) What are your hobbies?
(15) *What of these alternatives seems better? (limited choice indicated by the of-phrase, hence which is used: Which of these alternatives seems better.)
(16) What the hell/on earth/in the world are you saying?
(17) Who told Mary about the accident?
(18) Who are those people?
(19) *Who of your students will pass the exam? (limited choice indicated by the of-phrase, hence which is used: Which of your students will pass the exam?)
(20) Who the hell/on earth/in the world are you talking about?