@bylin At a recent semantics research group meeting in Tokyo, @Crime and other English native speakers reported that "Which student left" indeed presupposes that some student left, but "Which students left" is compatible with no student leaving. This would make sense if plural which-phrases include the "zero" extension while singular ones only range over singular extensions. Is this something that's been discussed in the literature?
@wataruu @Crime @bylin Huh, I don't get this judgement at all. The claim is that "which students left?" is a question that can be felicitously asked by a speaker who deems it possible that no students left???
Whatever the facts turn out to be, I have some work with Uli and Andreea on plural vs. singular simplex wh-phrases which might be relevant. We found that plural but not singular "who" carries an existence presupposition in languages with a morphological distinction.