Interrogative adjectives introduce questions by modifying a noun that is being identified, selected, or owned. They always come before the noun they modify, and they work with nouns of any number or gender because English does not change their form for agreement. They are closely related to interrogative pronouns, but an adjective stays with a noun while a pronoun stands alone.

Which and what both ask for identification, but which suggests a limited set of choices while what asks more generally. In informal US English, what can sometimes appear where which would be more typical in careful written English. When the noun is omitted, which one is a natural conversational form that avoids repetition.

Idea	Example
Which asks from a limited set.	Which book did you want?
What asks for general identification.	What color is the car?
Which one omits the noun when it is understood.	Which one did you choose?

Whose asks about possession and can modify nouns for people, animals, or things. With inanimate nouns, whose is acceptable in modern English, though some formal styles may sound cautious about it. The form does not change for singular or plural nouns.

Idea	Example
Whose asks about ownership.	Whose keys are these?
Whose can refer to things.	Whose idea was this?
Whose stays the same with number.	Whose tickets are missing?

An interrogative adjective always appears before the noun it modifies, so it occupies the same position as other determiners. It cannot normally occur with another determiner before the same noun, because English allows only one central determiner slot there. This is why forms such as which book or whose bag are grammatical, while two determiners before the same noun are not.

Idea	Example
The interrogative adjective comes before the noun.	Which answer is correct?
Only one determiner normally appears before the noun.	Whose notebook is on the desk?
The noun can be omitted when context makes it clear.	Which is yours?

Interrogative adjectives modify a noun, while interrogative pronouns replace the noun entirely. The choice between them depends on whether the noun is present in the question. This contrast is clear in pairs such as Which book is yours? and Which is yours?

Idea	Example
An interrogative adjective modifies a noun.	Which book is yours?
An interrogative pronoun stands alone.	Which is yours?
The noun may be omitted when it is already understood.	Whose are these?

Which is used when the speaker assumes a known set of possible answers, while what is used when the answer is open or general. Usage can vary by dialect and register, especially in informal speech, so careful context matters more than a rigid rule. These forms are part of the wider system of Determiners and behave like other noun introducers in English.