Why is -d used instead of -m for most neuter pronouns

Question

There is a notable set of pronouns that use -d for the neuter nominative and accusative:

• iste > istud
• ille > illud
• quis > quid
• is > id

Other pronouns do not:

• hic > hoc
• ipse > ipsum (though L&S lists once instance of ipsud)

My question is more in the realm of linguistics: Is there a reason why (only?) these pronouns use -d for the neuter? The answer may be as simple as pointing to the PIE derivative, though I'm also curious why this -d does not occur in neuter nouns of the second and (I believe) third declension.

Answer 1

There isn't really an answer to the "why" question beyond the fact that in Proto-Indo-European, some of the case endings for pronouns were different from those for nouns, for unknown reasons. Among these is the nom./acc. sg. neuter ending, which was *-d instead of *-m. This is clear from cognates in other languages, e.g. Sanskrit neuter demonstrative tad, neuter relative yad, etc. (The Greek cognates of these two forms are τό, ὅ, which also descend from *-d, since final stops were lost in Greek.)

Another specifically pronominal case ending, by the way, is the masculine plural *-oi, which in Latin (and in Greek) spread from pronouns to nouns, giving the familiar second-declension ending -ī.

Ipsum is presumably analogical on the nominal declension. Hoc actually did once contain the *-d: it's from *hod-ce, and in fact seems to have been pronounced hocc, which is why it scans long in verse.