An interrogative word or question word is a function word used to ask a question, such as what, when, where, who, why and how. They are sometimes called wh-words, because in English most of them start with wh-. They may be used in both direct questions (Where is he going?) and in indirect questions (I wonder where he is going). In English and various other languages the same forms are also used as relative pronouns in certain relative clauses (The country where he was born) and certain adverb clauses (I go where he goes).
A particular type of interrogative word is the interrogative particle, which serves to convert a statement into a yes–no question, without having any other meaning. Examples include est-ce que in French, ли li in Russian, czy in Polish, কি ki in Bengali, 吗 ma in Chinese and か ka in Japanese. (The English word whether has a similar function, but only in indirect questions.) Such particles contrast with other interrogative words, which form what are called wh-questions rather than yes–no questions.
For more information about the grammatical rules for forming questions in various languages, see Interrogative.
In English 
Interrogative words in English include:
- interrogative determiner
- which, what
- whose (interrogative possessive determiner)
- interrogative pro-form
Ultimately, the English interrogative pronouns (those beginning with wh in addition to the word how), derive from the Proto-Indo-European root kwo- or kwi, the former of which was reflected in Proto-Germanic as χwa- or khwa-.. In how (Old English hū, from Proto-Germanic χwō), the w merged into the lave of the word, as it did in Old Frisian hū, hō (Dutch hoe "how"), but it can still be seen in Old Saxon hwō, Old High German hwuo (German wie "how"). The Proto-Indo-European root directly originated the Latin and Romance form qu- in words such as Latin quī ("which") and quando ("when"). In English, the gradual change of voiceless stops into voiceless fricatives (phase 1 of Grimm's law) during the development of Germanic languages is responsible for "wh-" of interrogatives. Although some varieties of American English and various Scottish dialects still preserve the original sound (i.e. [hw] rather than [w]), the majority only preserve the [w]. The words who, whom, whose, what and why, can all be considered to come from a single Old English word hwā, reflecting its masculine and feminine nominative (hwā), dative (hwām), genitive (hwæs), neuter nominative and accusative (hwæt), and instrumental (masculine and neuter singular) (hwȳ, later hwī) respectively. Other interrogative words, such as which, how, where, whence as well as the now archaic whither derive either from compounds (which coming from a compound of hwā [what, who] and līc [like]), or other words from the same root (how deriving from hū).
Forms with -ever 
Most English interrogative words can take the suffix -ever, to form words such as whatever and wherever. (An older form of the suffix is -soever, as in whomsoever.) These words have the following main meanings:
- As more emphatic interrogative words, often expressing disbelief or puzzlement in mainly rhetorical questions: Whoever could have done such a thing? Wherever has he gone?
- To form free relative clauses, as in I'll do whatever you do, Whoever challenges us will be punished, Go wherever they go. In this use, the nominal -ever words (who(m)ever, whatever, whichever) can be regarded as indefinite pronouns or as relative pronouns.
- To form adverbial clauses with the meaning "no matter where/who/etc.": Wherever they hide, I shall find them.
Some of these words have also developed independent meanings, such as however as an adverb meaning "nonetheless"; whatever and whatsoever as emphatic adverbs used with no, none, any, nothing, etc.; and whatever in its slang usage.
Other languages 
A frequent class of interrogative words in several other languages is the interrogative verb:
- Korean: Nalssi-ga eotteoh-seumni-kka? (Hangul: 날씨가 어떻습니까?) Weather-nominative be_how-politeness fifth level-interrogative suffix "How's the weather?"
- Mongolian: Chi yaa-vch jaahan huuhed bish gej bi bod-jii-ne You do_what-concessive small child not that I think-progressive-nonpast "Whatever you do, I think you’re not a small child." (Example taken from an Internet forum)