(13) Structure and position of QuP (from Aoun and Li 1993: 235)
Leaving aside the technical details, this contrast can be accounted for if we assume that temporal adverbs are adjoined to a syntactically lower category and epistemic adverbs to a higher category than QuP. In (15) both question forms can take the adverb under their scope, whereas in (16) only the particle question, which is constructed by a particle in the highest position in C, can express a well-formed question.