Not to belabor on the subject, but the Classic Greek's infinitive has only case (it is always singular and neutral in gender), and the Latin infinitive has no gender, number, or (essentially) case.
So it is unclear to me, what "Western grammatical tradition" compelled them to assume that the inflection of the Hindustani infinitive stops at the gender.
I guess the aim was just to "keep the infinitive accidence as simple as possible".
I'm basically just re-iterating what
@Dib has already said in posts above, but...
Let's make a distinction between the
morphological form of a verb and the
function of that form. The morphological form in question here are the -naa(/-ne/-nii) forms. This form can unequivocally function as a verbal noun. What
@Dib pointed out in post #8 was that this form can also function as a verbal adjective. Platts/Naim call the form-function pair consisting of the -naa form with its verbal noun function an "infinitive," and the form-function pair consisting of the -naa form with its verbal adjective function a "gerundive."
The reason this gets confusing, I think, is that the -naa morphological forms are often also called "infinitives" (as opposed to the Platts/Naim usage of the word "infinitive," which refers not just to a morphological form but, again, to a form-function pair). It's perhaps this double meaning of the word "infinitive" that's confusing you,
@MonsieurGonzalito...? For the remainder of this post, let me avoid this latter usage of "infinitive." I'll refer to the morphological form as the "-naa form," and use "infinitive" and "gerundive" to indicate form-function pairs.
In a sentence like
mujhe kapRe dhone paRenge, Platts/Naim deem
dhone a gerundive. This fits with analyzing constituents as in #9, and another nice observation
@Dib made above in #14 was that the
dhone here can actually be replaced by other adjectives (eg,
mere liye kapRe mahange paRenge, meaning roughly, "The clothes will be expensive for me.") Note that the gerundive (ie, the -naa form functioning as a verbal adjective) displays both gender and number agreement: it's inflected to show masculine plural agreement in mujhe bartan dhone paRenge, and we have
mujhe kitaab paRhnii paRegii ("I'll have to read the book"), etc.
In other words, Platts/Naim would probably not disagree with the sentence "the -naa form can be inflected for plural agreement," but they'd throw in the caveat that it must be a gerundive. When Platts/Naim say that there is no plural infinitive, they are talking specifically about the the -naa form functioning as a verbal noun. And this brings me to the next point...
If #6 is grammatical, then I would tend to take it as a plural infinitive indeed, unless it is to be understood as: "(jaise) qismat meN likhe the, (beTaa) bartan dhone (nikle)", in which case it would be an oblique singular infinitive. However, I am not confident enough to judge the grammaticality of the plural infinitive interpretation. I'd like to ask other forum members to help.
I think I'm reaching the point where I've thought about this issue too much in a short span of time because I'm now having doubts about all of my grammaticality judgments about this issue... But let me proceed anyway, realizing that I'm probably well at risk of others telling me that my judgments are incorrect
The plural infinitive interpretation of #6 sounds fine to me. It also seems to me that
qismat meN kapRe dhone likhe the is basically synonymous with
qismat meN kapRe dhonaa likhaa thaa. Neither of these sounds awkward to me (in contrast, I find it slightly jarring whenever I run into the usages of style 1 and 2 from
@Qureshpor jii's very thorough post #15, even though they certainly occur frequently!). This suggests that the phenomenon of infinitives (ie, -naa forms functioning as verbal nouns) inflecting to show gender/number agreement is in relatively free alternation with masculine singular infinitives.
I also feel like I may have been too unequivocal in post #11. Probably
merii qismat meN kapRe dhone hii the would also be okay...? That being said, I still feel like just a bare "kapRe dhone" would be a slightly weird response to A's question, so I'm not sure what to make of that as far as constituent analysis goes... Perhaps there are other, more reliable, constituency tests.
@marrish saahib gave us a nice historical example in post #4 (roughly,
mu'allif chand ash3aar likh dene munaasib jaantaa hai), and I'm slightly uncertain about the explanation of this as an oblique in post #12. It seems to me that this is also a plural infinitive, and that it would be in free alternation with
mu'allif chand ash3aar likh denaa munaasib jaantaa hai. I'd be interested to hear someone expand on the oblique explanation in #12, if possible: what reasons are there to regard it as an oblique...?