In Noun phrases^x, Expressions^x, Articles^x

All of your examples are gerunds if we stick to modern terminology. Forget about the terminology verbal noun . It's total garbage! It has many different definitions, depending on the author and when the grammar book was written. __ The modern definition is given at ...

"Air-borne" is "carried by aircraft" or "carried through the air". Expressions using "airborne": ♦ airborne alert ♦ airborne assault ♦ airborne command post ♦ airborne equipment ♦ airborne force ♦ airborne force liaison officer ♦ airborne forces ♦ airborne interception equipment ♦...

Actually, "nonsense" (as well as "wood", "knowledge", etc.) are all singular noncount nouns. Maybe the question should not be when to use "what" vs. "what a". Maybe it's just a matter of saying that, in this type of expression, "what" precedes whatever the usual indefinite noun phrase normally...