We use interrogative pronouns to ask questions. The interrogative pronoun represents the thing that we don't know (what we are asking the question about).
There are four main interrogative pronouns: who, whom, what, which
Notice that the possessive pronoun whose can also be an interrogative pronoun (an interrogative possessive pronoun).
Notice that whom is the correct form when the pronoun is the object of the verb, as in "Whom did you see?" ("I saw John.") However, in normal, spoken English we rarely use whom. Most native speakers would say (or even write): "Who did you see?"
Look at these example questions. In the sample answers, the noun phrase that the interrogative pronoun represents is shown in bold.
Note that we sometimes use the suffix "-ever" to make compounds from some of these pronouns (mainly whoever, whatever, whichever). When we add "-ever", we use it for emphasis, often to show confusion or surprise. Look at these examples: