Interesting Master thesis Enigmatic *-nt-Stems: an investigation of the secondary -t- of the Greek neuter nouns in *-men- and *-r/n-, by Stephanie Stringer, Université de Montréal (2018).
This paper aims to provide an explanation of the secondary -t- found in the oblique stem of ancient Greek neuters such as πρᾶγμα, πράγματος and ἧπαρ, ἥπατος. After a brief overview of the Greek data, and a survey of the relevant nominal classes in Greek and Indo- European, previous hypotheses are evaluated. To this end, several problems of nominal morphology are discussed, including the existence of a PIE suffix *-m(e)ntom, the secondary -t-s of certain animate nouns, the ablatival suffix *-tos, the Hittite ergative; and the ablaut of neuter active participles. Certain phonological issues are also addressed. Since the majority of hypotheses formulated to explain the secondary -nt- inflection of Greek neuters date from the nineteenth century, attempts are made to re-evaluate their conclusions in the light of more recent research, particularly that related to ablaut classes. Also considered are a number of twenty-first century works which purport to explain the Greek data as part of a larger Indo-European phenomenon.
This paper makes no attempt, however, to explain the PIE origins of either the *r/n-, or of the *nt- stems. It concludes that the best explanation of the Greek declensional pattern is to be found in the analogy between stems in -nt- and those in *-mn- or *-r/n-.
Interesting excerpts, from the conclusion (emphasis mine):
In comparison with other proposed solutions, there are relatively few objections to be levelled at Schmidt’s theory, in this slightly modified form. Anghelina (2010) criticises it on the ground that it does not provide an explanation of how the -t- came to be inserted into the r/n-stems, but this criticism has already been addressed. Anghelina also objects to the idea that participles might affect the declension of nouns. Given that participles can function syntactically as nouns, and that their declension is formally identical, except for the distinction of gender, it is difficult to see why the inflection of one might not affect the inflection of the other. Furthermore, it appears to have done so within the history of Greek. In addition to the neuters, a number of masculine n-stems are inflected as nt-stems, although related formations within Greek attest to the secondary nature of the -t-, e.g. δράκων, -οντος, but δράκαινα, λέων, λέοντος, but λέαινα etc. Perhaps Anghelina would prefer to explain these cases also as
developments from the dative plural, before the ablaut was levelled, but in general, the influence of participles is accepted as an explanation. One could also point to the influence of the pronominal declension on the endings of thematic stems in PIE.
Sihler (2008, 297) argued that if one accepted the nt-stems as a model it was “hard to progress beyond a vague likelihood” and “the supposed model paradigm has been everywhere replaced.” The first of these criticisms is valid, in a sense. One cannot conclusively demonstrate that the nt-stems served as the model for the men- and r/n-stems. Only that the model was available, and that the outcome of the change conforms with it. However, it does not seem that Sihler’s caution is more pertinent in the case of this theory than in any other proposed explanation of morphological change.
Silhler’s second criticism, is true as well. The starting point, a NA sg. nt. -n̥, G sg. -n̥t-os is indeed only preserved, at best, in a few relics. All the same, it can be assumed with some confidence to have existed at the right time. Furthermore, the analogy is unobjectionable. The nt. NA sg. ending -n̥ could genuinely belong to an nt-stem as well as to an n-stem. It is no surprise that the neuters were systematically replaced, while the masculines and feminines showed only sporadic transition to the nt-declension. In the neuter both the NA were liable to re-interpretation as a t-stem, while in the animate forms only the nominative was. The m(e)n-stems are a highly uniform group, and it is easy to understand how a change could spread relatively quickly. The connection to the r/n-stems is slightly more tenuous, but they do have more in common with the m(e)n-stems than with any other group. (A m(e)r/m(e)n- suffix does exist, but given that it is quite rare, and given that its only two representatives in early Greek, τέκμαρ and τέκμωρ are attested only in the NA sg., it is hard to see that this subclass can have played a significant role.)
The nt-stem theory provides an adequate explanation of the Greek situation. That was indeed the very limited aim of this paper. In very general terms, it may also provide an explanation for some of the “stray” t’s one finds attached at times to n-stems in PIE or other languages. Given the co-existence, whatever their origin, of both -nt- and -n-, and in fact t-stems, and given that both -n- and -t- were under certain conditions liable to be lost or assimilated to surrounding sounds, one might expect to find a certain degree of erratic fluctuation between the two classes. Such an observation is so vague as to be quite unhelpful, but at least it is not contradicted by known facts.
In opting for a solution that seems to account for the facts in Greek, one is forced to leave many other phenomena unexplained. Although it would be more satisfying if one were able to draw together the -t- of the NA *-r/n- in Sanskrit and the -t- of the nearly synonymous suffixes -man-, -vant-, man, mant, vasanta, gimmant- etc., it seems at present they can only be connected if one ignores many of the details of each specific situation. For the time being, it appears they must be dismissed as similar, but essentially unrelated, or at least only very indirectly related phenomena. It is entirely possible that further research will reverse this conclusion.