Interrogative pronouns (who, whom, what, which, whose) introduce questions and ask for information about people, things, or possession. 'Who' and 'whom' ask about people ('Who is that?' 'Whom did you see?'), 'what' asks about things ('What is that?'), 'which' asks for a choice ('Which one do you prefer?'), and 'whose' asks about possession ('Whose book is this?'). Each interrogative pronoun has a specific function and cannot be substituted for another.
You already know that pronouns stand in for nouns, and that questions in English typically invert the subject and auxiliary verb. Interrogative pronouns build on both: they replace the noun you're asking *about*, and they typically sit at the front of the sentence — before the inverted word order begins. The result is a question word that simultaneously names the gap ("I don't know who/what/which X") and triggers the inversion ("did you see" rather than "you saw").
The easiest distinctions are between what and which. Use *what* when the range of possible answers is open: "What did you eat?" could be answered with any food in the world. Use *which* when you're choosing from a defined or implied set: "Which dish did you prefer — the pasta or the salad?" The constraint built into *which* is why it implies selection. A teacher asking "Which president first signed the bill?" implies there is a correct answer from a known list; "What do you think of the bill?" opens to any opinion.
Whose asks about possession and is the interrogative form of the possessive. "Whose jacket is this?" = "This is [someone's] jacket — whose?" It is one of the most commonly confused words in English because it sounds identical to "who's" (who + is/has), but they do very different jobs: "Whose jacket?" asks about ownership; "Who's coming?" asks about identity with a contraction.
The hardest pair is who vs. whom, which maps directly onto the pronoun case you've been studying. *Who* is the subjective form (like *he* or *she*); *whom* is the objective form (like *him* or *her*). The classic test: answer the question mentally. "Who called?" → "He called." (Subjective — use *who*.) "Whom did you call?" → "You called him." (Objective — use *whom*.) Because questions invert word order, the pronoun often appears far from its grammatical slot, which is why the substitution test ("he/she" vs. "him/her") is so useful. In formal writing and speech, the distinction still matters — in casual speech, *who* is increasingly used in both positions, but recognizing the underlying logic is what lets you choose deliberately rather than guess.