quality:corruption
Interrogative

The interrogative pronouns are who, what, and which (all of them can take the suffix -ever for emphasis). The pronoun who refers to a person or people; it has an oblique form whom (though in informal contexts this is usually replaced by who), and a possessive form (pronoun or determiner) whose. The pronoun what refers to things or abstracts. The word which is used to ask about alternatives from what is seen as a closed set: which (of the books) do you like best? (It can also be an interrogative determiner: which book?; this can form the alternative pronominal expressions which one and which ones.) Which, who, and what can be either singular or plural, although who and what often take a singular verb regardless of any supposed number. For more information see who.

In Old and Middle English, the roles of the three words were different from their roles today. "The interrogative pronoun hwā 'who, what' had only singular forms and also only distinguished between non-neuter and neuter, the neuter nominative form being hwæt. " Note that neuter and non-neuter refers to the grammatical gender system of the time, rather than the so-called natural gender system of today. A small holdover of this is the ability of relative (but not interrogative) whose to refer to non-persons (e.g., the car whose door won't open).

All the interrogative pronouns can also be used as relative pronouns, though what is quite limited in its use; see below for more details.