[seanhunter]

It’s a nice analogy but Latin has tons of weird idioms and exceptions. Been a while since I did it, but

1)the locative vs the ablative, and the locative only existing for a few words

2)the irregular verbs such as sum, eo etc, irregular nouns such as deus, aqua etc, and there’s a bunch of irregular like adjectives and stuff that I don’t remember

3)indeclinable nouns that just don’t decline at all and are the same in all cases. I think the word for “morning” is like this but it’s been a very long time. There are a few words that work this way anyway.

4) Words like “castrum” which just mean something totally different in the plural to the singular. “Castrum” means a fort, but the plural “castra” doesn’t mean many forts, it means a (singular) military camp.

5) Words like “Saturnalia” (festivals of Saturn) which only exist in the plural. As far as I know you can’t say one festival of Saturn in latin.

[schoen]

For (3), the word you're thinking of is "mane" 'in the morning', which looks very much like an ablative but which doesn't have any other forms. There are definitely other words like that, such as "fas" 'right, propriety, justice'.

For (5), these are called pluralia tantum (singular "plurale tantum").

https://en.wikipedia.org/wiki/Plurale_tantum

I gave some examples of Latin irregularities elsewhere in the thread, and I like your examples too!