Different Interrogative words function to target different parts of a sentence. Interrogative pronouns replace and target a noun (what and who), whereas interrogative adverbs replace and target a constituent with an adverbial function, such as when (adverbial of time), where (adverbial of space). To ask a content question, one simply replaces the constituent one wants information about, for instance the subject (who is sleeping in my bed?), or the place (where did you sleep?). The question is then, how does one go about to ask a question about the verb of a sentence?
In English, this is done in rather complicated way. One has to use to interrogative pronoun what together with the verb do, as in ‘what are you doing?’. There is no word that can directly replace the verb, potentially leading to mismatches between the structure of the question and the structure of the answer, unlike the example above. For instance, ‘what are you doing’ is a transitive construction (it has an object, what), even though the answer might be intransitive (I was sleeping).
There are, however, languages that do not suffer from this awkward way of posing verbal questions, languages that can directly target and replace the finite verb of a sentence. To have a look at some of them, we shall follow Olof and Loren and take a closer look at the Austronesian language family, where we can find languages with not only interrogative pronouns and adverbs, but also interrogative verbs, (interrogative proverbs might be a better term, in analogy with interrogative pronouns, in which case we really should use the term interrogative proadverbs as well!).
The interrogative (pro-)verb marhua in Toba Batak (spoken on Sumatera), which literally means ‘do what’, can provide an illustrationg of how they work. Marhua can be used as the finite verb of a sentence, as in ‘marhua(do.what) ibana(he) disi(here), meaning ‘What is he doing here’, or more literally, ‘whated he here?’. Just like ordinary verbs in Toba Batak, the interrogative verb stands in the beginning of the clause, and it has the prefix mar-, which is a verbal prefix. The word is not only a (pro-)verb because it replaces a verb, but also because it has the same morphology as verbs, and is located in the same syntactic position.
While interrogative (pro-)verbs are not unique to Austronesian languages, they are found throughout the language family. Some other examples include Cebuano (spoken in the Philippines) with the interrogative (pro-)verb magunsa, Tongan (spoken on Tonga) with haa, Makassar (spoken on Sulawesi) with anapa, Colloquial Indonesian with ngapain, Bunun (spoken on Taiwan) with makua, and Palauan (spoken on Palau) also has an interrogative (pro-)verb mekera. All of these interrogative (pro-)verbs have the meaning ‘do what’, and can function as the finite verb of a sentence to inquire information regarding what someone did.
Like in Toba Batak, all the interrogative verbs above can host verbal morphology: ng- in Indonesian, an- in Makassar, ma- in Bunun and mag- in Cebuano, are all affixes that normally are situated on verbs. Interrogative verbs not only function as finite verbs, but they also have the same morphological (the have verbal affixes) and syntactic properties (they take the position of a verb in the clause) as lexical verbs!
Interrogative (pro-)verbs is a relatively poorly understood linguistic phenomenon, as there to date has been little research on it. Exploring rarer linguistic phenomenon like interrogative verbs is important as it allows us to get a better understanding of both the possibilities and limitations on variation in human language.