(This answer is based on Weiss's Historical and Comparative Grammar of Latin and Clackson and Horrocks's Blackwell History of the Latin Language.)
The first thing to know about these two ablative endings, -e and -ī, is that neither of them is descended from the Proto-Indo-European (PIE) ablative ending.
-e comes most probably from the PIE locative ending, *...
Tuomo Pekkanen's Ars Grammatica (a Latin grammar in Finnish) says that the second declension has three neuters ending in -us: vīrus, vulgus and pelagus.
They are only used in the singular, and accusative is exactly like the nominative (not -um).
I have no clue about the origin of these words.
I'm not sure if these words even have a similar history.
It came to Latin from Hebrew (שָּׂטָן satan), through Greek (Σατανᾶς satanas) and means enemy, adversary.
In Judaism and Christianity, it is also one of the names given to the devil, a supernatural creature that lead a rebelion against God and one of the main instigators of evil in the World.
The -as ending is purely grammatical. As can be seen, it was ...
Leumann (p. 421) mentions two cases:
spoken gen.pl. drachmum and amphorum;
in dactylic poetry, four-syllable masculine nouns, besides the regular forms, could also have gen.pl. in -um, mostly compounds with -cola and -gena (e.g. agricolum in Lucr. 4.586 or caelicolum; Troiugenum), and some Greek proper nouns (Gangaridum, Aeneadum, Phaselitum).
Here’s a summary of what most authoritative Latin grammars say on the genitive singular ending of –io stems (Weiss 2009/2011: 222-223; Leumann 1977: 424-425; Sihler ). For the sake of simplicity and consistency, in my answer I use the periodization of Latin as used in Weiss (which is different from, for instance, Clackson and Horrocks or Meiser). Weiss has ...
(This answer is based on Weiss's Historical and Comparative Grammar of Latin, which is usually the place to go for this kind of thing.)
The most common Indo-European 3pl active ending is -nt(i), which is part of the familiar set -m(i), -s(i), -t(i), ..., -nt(i). (The 1pl and 2pl are a bit harder to reconstruct because they vary more in the individual ...
Bennett gives gen. alterius, dat. aliī. Allen and Greenough list alius among the adjectives that "have the Genitive Singular in -īus and the Dative in -ī in all genders", implying alīus, aliī, but add in notes that "Instead of alīus, alterīus is commonly used" and that "The regular genitive and dative forms (as in bonus) are sometimes found in some of these ...
Edgar H. Sturtevant's dissertation "Contraction in the case forms of the Latin io- and ia stems, and of deus, is, and idem" (1902) seems to have some relevant info, although I don't know if more has been discovered since then.
Contraction in the genitive singular
Sturtevant starts out by summarizing the genitive singular forms: he says that in early Latin, ...
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, ...
Gildersleeve and Lodge, §76.r1: The Gen. alīus is very rare, and as a possessive its place is usually taken by alienus.
§76.r2: …usually make the Dat. Sing. in -ī … Alī is found in early Latin for aliī.
A full table of "standard" (post-Augustan) -ius/-ium endings would be:
M SG M PL N SG N PL
NOM -ius -iī -ium -ia
GEN -iī -iōrum -iī -iōrum
DAT -iō -iīs -iō -iīs
ACC -ium -iōs -ium -ia
ABL -iō -iīs -iō -iīs
VOC -ī -iī -ium -ia
(O tempora! O mores! Why must upstanding citizens be ...
Lateinische Grammatik (Leumann, Hofman and Szantyr 1977) argues that
"Im Genetiv ist -īus die Standardform der Endung, sie gilt für Plautus und für die klassische Prosa verbindlich. In metrischen Texten bestehen zwei Nebenformen" (Para 376B2a, p. 479) [emphasis mine - Alex B.]
They also add that "Einigermassen häufig sind nur gen. -i und dat. fem. -ae" (...
This answer is based on the intuition I have long held: genitives like nostrum and vestri are not forms of the personal pronouns nos and vos, but of the substantivized possessive adjectives.
I have no sources to back this up, but I am posting these thoughts here for scrutiny.
Some pronouns (ego, tu, nos, vos, se) have an associated possessive pronoun (meus, ...
I don't think we can know for certain, but the fact that the spelling "filii" occurred makes it likely that the word was perceived as having three syllables, and was so treated in poetry. There are various possibilities of how it was actually pronounced:
1) A single extra-long vowel, [i::], in which case the spelling represents perception only.
2) It's ...
It's all too easy to confound the reflexive pronoun (which only exists in the oblique cases) with the possessive adjective suus, sua, suum (declined like bonus). The genitive plural suorum , etc, of the latter is found widely in classical literature, which I personally find more readily acceptable than the singular usage of the reflexive pronoun.
I'm not ...
Just to clarify:
The -as here is actually the Greek masculine ending -ᾱς. It's a dialectal variation on the -ης (-ēs) you see in names like Socrates, Euripides, Achilles, and so on: probably the most common ending for male names in Ancient Greek.
So when people started translating the Bible and the Torah into Ancient Greek, and came across the word שָׂטָן (...