It is the WH- element that expresses the nature of the missing piece: 'who', 'what', 'when', 'how' etc. So in a WH- interrogative the WH- element is put first no matter what other function it has in the mood structure of the clause, whether Subject, Adjunct or Complement. The meaning is ʻI want you to tell me the person, thing, time, manner, etc.ʼ. Interrogative clauses, therefore, embody the thematic principle in their structural make-up. It is characteristic of an interrogative clause in English that one particular element comes first; and the reason for this is that that element, owing to the very nature of a question, has the status of a Theme. The speaker does not choose each time to put this element first; its occurrence in first position is the regular pattern by which the interrogative is expressed.
- Halliday (1985), a pag.47

The WH- element is a distinct element in the interpersonal structure of the clause. Its function is to specify the entity that the questioner wishes to have supplied. The WH- element is always conflated with one or another of the three functions Subject, Complement or Adjunct. If it is conflated with the Subject, it is part of the Mood element, and the order within the Mood element must therefore be Subject ^ Finite [...] If on the other hand the Wh- element is conflated with a Complement or Adjunct, it is part of the Residue; and in that case the typical interrogative ordering within the Mood element reasserts itself, and we have Finite preceding Subject [...].
- Halliday (1985), a pag.83