Some authors recommended differentiating between plural imperatives and deverbal nouns by writing the latter without hamza like پھیلاو 'extent' but پھیلاؤ 'spread!'. Despite I believe Rasheed Hasan Khan's opinion it didn't gain traction.
Just to flesh out the reference in case anyone (else) gets curious, this opinion occurs in Rasheed Hasan Khan's 1975 book
اردو کیسے لکھیں (pages 62-64, with related remarks on page 49).
But this is English grammar. Would you apply it to Urdu and designate پھیلاؤ as a deverbal noun?
To the extent that one wishes to distinguish the terms "verbal noun" and "deverbal noun," it feels to me like marrish jii's use of "deverbal noun" was spot on!
My understanding is that, to the extent that the two terms are distinguished, "verbal noun" is used for a systematic morphological form of a verb that is used to create phrases that behave like nouns, while "deverbal nouns" might derive etymologically from verbs, but they do so in idiosyncratic, non-systematic, and lexicalized ways. For example, in a sentence like
is biimaarii kaa phailnaa kab band ho_gaa?, it is the -
naa form that is used to create the noun-like phrase
is biimaarii kaa phailnaa which then serves as the subject of the sentence. Basically every verb systematically has this -
naa form, so this
phailnaa can be said to be a "verbal noun." On the other hand, the
-aa'o/-aav suffix that appears in
phailaa'o/phailaav is not completely systematic verb morphology in the same way as the -
naa form: not all verb stems can be appended with this suffix to make something that behaves like a noun. For example, as far as I am aware, one does not typically form a
noun ?
muskuraa'o/muskuraav from the verb stem
muskuraa-, and one normally uses
muskuraahaT instead. Linguists seem to speak about "deverbal nouns" in a huge wide variety of genetically unrelated languages (eg,
Japanese,
Mandarin,
Swahili, ...), and other languages also have a variety of idiosyncratic ways of forming deverbal nouns from verb stems, so it feels to me like using the same terminology here is fine.