Interrogative Adjectives

English has only three interrogative adjectives—what, which, and whose—but they carry an enormous amount of work in everyday questions. Each one modifies a noun to ask for identification, selection, or ownership: What time is it?, Which route should we take?, Whose bag is this? Unlike Demonstrative Adjectives, which point to something already known, interrogative adjectives signal that the identity of the noun is exactly what the speaker wants to discover.

The choice between what and which depends on the size of the set you are choosing from. Use what when the options are unlimited or unknown: What colour do you prefer? implies the entire spectrum of colours. Use which when the options form a closed, known set: Which colour do you prefer—red or blue? This distinction is not always enforced in casual speech, but getting it right in formal writing adds precision.

Whose is the possessive interrogative adjective and asks about ownership. A widespread spelling error is confusing whose with the contraction who's (short for who is or who has). In Whose phone is ringing?, the adjective modifies phone to ask about possession. In Who's calling?, the contraction means Who is calling. If you can replace the word with who is and the sentence still works, you need the contracted form—otherwise, it is the possessive adjective whose.

Interrogative adjectives appear not only in direct questions (What book are you reading?) but also in indirect questions (I wonder what book she is reading) and reported questions (He asked me which train I had taken). In indirect and reported forms the word order shifts from question order to statement order, and the question mark disappears—a subtle shift that many intermediate learners overlook.