Templum is a neuter noun of the second declension. These end in -um in the nominative.
Note these two important rules about neuter nouns:
all neuter nouns have exactly the same form in nominative and accusative (No exceptions exist whatsoever)
all neuter nouns end in -a in the nominative and accusative plural. Again, no exceptions ever.
I think it's wrong for "All neuter nouns have exactly the same form in the nominative and accusative"
"All neuter nouns end in -a in the nominative and accusative plural.
The wrong part is "no exception, no exception ever".
Epulum is neuter, nominative singular,
and means: feast, banquet.
epulum (nom, sing)-> epulae (nom, plur.)
Yes, it's the feminine declension, but the noun is still neuter. So, an exception to the "same form with -a in nominative and accusative plural.
And the rule "they have exactly the same nominative and accusative" is wrong, for the plural here also.
nom. sing epulae
nom. plural epulas
Epulum is an example of neutrer noun with weird declensions, belonging to several groups. They call it an heterogenous noun.
Heterogenous neuter nouns: have neuter 2nd declension when they are singular. And they have feminine 1rst declension when they are plural.
Yes, it's true that they say that in Late Latin, the weird forms of "epulum" disappeared, and it was all a regular 2nd group of declension noun,
but I've found other exceptions as pascha, accidens, etc... I don't know if they became all regular declensions nouns in Late Latin.
Some exceptions do exist.
You say "Yes it's the feminine declension, but the noun is still neuter."
In the singular, "epulum" is neuter, but in the plural, "epulae" is an actually feminine noun and takes feminine agreement, e.g. "funestās epulās fratrī comparāre" (Cicero).
See also 106.b in
"epulum (n.) feast. epulae (f.) feast"
Lewis and Short also describe it as neuter in the singular and feminine in the plural.
The Neuter Plural Rule does not apply to "epulae", since "epulae" is not neuter. A feminine noun cannot be an exception to the Neuter Plural Rule.
'Altar' is a later form of 'Ara', in other words, 'Altar' is Late Latin and 'Ara' is Classical Latin. Ecclesiastical Latin, which is what the Catholic Church uses for The Extraordinary Form Always and Ordinary Form sometimes, among other things like Papal Encyclicals and Official Church Documents, is largely Classical Latin, but with Vulgar and Late Latin words, said with an edge as if you're speaking Italian (Mostly 'V' is still 'V' and not 'W' like Classical usually does and 'AE' is an enlongated and somewhat stressed verse of 'A' instead of being said almost like an E/I sound like in Classical Latin, and all the other Dipthongs are 'A-' with the same rules), along with some of the words having a somewhat more religious edge to them. By and large, it's the same langauge and one can understand the other with not much difficulty if he can comprehend one version of the language