Will someone please explain to me the difference between plurals ending in “i” versus those ending in “ia”? E.G., millennia vs. millenni?
millennium = second declension, neuter → millennia
compare a second-declension masculine noun: dominus → domini
In short, the -a plural is for neuter nouns.
-ia is not a Latin plural ending. In “millennia”, the “i” is part of the root, and stays part of the root. The word is “millenni + um”. And the roots don’t change, no matter what happens to the endings.
Most Latin nouns fall into one of three categories, called “First Declension” (which is usually feminine), “Second Declension Masculine”, and “Second Declension Neuter”, and each has its own pluralization rules.
First Declension words all end in -a in the singular (technically, in the nominative singular, but that’s a distinction that seldom matters for English speakers), and in -ae in the plural. For instance, a female graduate is an alumna, and a group of them are alumnae. A single insect feeler is an antenna, and two of them are antennae.
Second Declension Masculine words all end in -us in the singular, and in -i in the plural. So for example, “radius” becomes “radii” (one i is part of the root, and the other i is the plural ending).
Second Declension Neuter words all end in -um in the singular, and in -a in the plural. Examples here include one millennium, multiple millennia, or one bacterium, many bacteria, or one vacuum, many vacua.
There’s also a third declension, but it’s not as consistent. Third declension plurals are always the root + es, but the nominative singular can be almost anything (that one specific form may or may not use the same root as all the other forms). The most common pattern is -is in the singular, such as one penis, or multiple penes. But you can also have, for instance, “rex” (king) and “reges” (kings). And if for some reason you needed a plural for “Jupiter”, it would be “Joves”.
And just for the sake of completeness, there are also a fourth and fifth declension, but they’re really rare even in Latin, and I can’t think of any English loanwords from them, so don’t worry about those.
ETA I noticed the question was specifically about -ia; this is a legitimate plural ending for neuter nouns with a pure i-stem
https://dcc.dickinson.edu/grammar/latin/3rd-declension-pure-i-stem-n
mare → maria
animal → animalia
Of course the “i” is part of the root, as you point out.
Some dope Doper once wrote a Straight Dope Staff Report on Latin plurals for the uninitiated: What is the plural of “penis”?
Most nouns are not any of those. The majority are third declension. There are more third declension nouns than all the first and second ones (and fourth and fifth) combined.
How does “Romanus eunt domum” parse?
Huh, I stand corrected, then. I suppose that my thinking otherwise is an artifact of my education, where you see more first and second declension nouns just because they’re simpler and taught earlier.
Scraping the rust off my school Latin - it doesn’t.
Romanus is singular, eunt is plural (=they go/are going), domum is a singular accusative, or an adverb meaning “to home”.
So if you want “Romans go home”, you need plural subject and plural imperative verb: Romani ite.
And the vocative case for “Romans” and locative for “home”, of course.
Now write that on the wall 1000 times, by morning.
[sound of a sword unsheathing]