"I saw the geniuses' octopuses," says person one.

"No, you saw the genii's octopi!" retorts person two.

"Ha! Wrong again!" says person three. "It's octopodes, not octopi!"

You know people like this. Maybe you are a person like this.

Well, guess what. Still wrong.

Isn't genii the Latin plural of genius and octopodes the Latin plural of octopus

Sure. The nominative plural. That's used for the subject of a sentence. There are different forms if it's the direct or indirect object or if it's possessing. Look, these are loan words, so we can always treat them like English words: geniuses', octopuses. But if we're going to insist on using the plural from the source language, don't you think we're being inconsistent if we don't also use the appropriate forms for the possessive — and for when it's the direct or indirect object? 

You see, if you say genii's, you're using one Latin ending and one English ending. (Not only that, you're using the English singular possessive ending on a plural noun.) The Latin possessive plural (it's actually the genitive case) is geniorum. So it's the geniorum octopodes.

But wait, there's more. Latin words can have different forms when they're the object rather than the subject — the accusative case rather than the nominative. The accusative plural of octopus is still octopodes, but in the singular it's octopodem. In fact, if there's just one genius and one octopus — the genius's octopus — it's the genii octopodem, because the genitive singular of genius is also genii.

We do this kind of thing with pronouns in English: I give him your book, not I give he you book (or you's book). Some other languages do it for all nouns. Because we don't, we think that if you use the nominative plural, you're covered. But that's just exoticist tokenism. Want to show some real respect? Here's a starting list of some words to get right.

appendix, matrix

One appendix, two appendices. One matrix, two matrices. That's all fine as long as they're the subject of the sentence. But I don't have an appendix, I have an appendicem. I can't be in the matrix, I have to be in matrice (that's the ablative case!). And if it's plural, well, not in appendices but in appendicibus! What's more, I can't give the appendix the matrices' contents; I need the dative and genitive — I give the appendici the matricum contents. 

Yes, yes, it's true, the Latin case system drove British schoolboys insane for centuries and may have been a cause of several wars (just to get away from it). But do you want to do it right or not?

blini

If you pay attention you know you don't have a blini. You have a blin. Only you can't eat just one, so yes, you have two blini. As it happens, the accusative forms of this Russian word are the same as the nominative ones. Fine, but you do not spread jam on your blin or your blini. Spreading jam on something requires the dative. And the dative is blinu (one) or blinam (more than one). You also use the dative in Russian with the verbs that mean "like" and "need." So "I don't need just one blinu, I need six blinam!" Genitive? If it's one, it's blina; more than one, blinov. So not "the blinis' filling" but "the blinov filling." And there are also instrumental and prepositional cases. Better get studying!

criterion

Some people take great pains to make sure you understand that you can't have one criteria. It's one criterion. That's all well and good, but do they tell you that it's not one criterion's, it's one criterii? Wait, it may have come by way of Latin but it's originally Greek. Maybe we should say one criteriou. And not several criterions' but several criteriorum if you go with the Latin, or several criterion (long o, though!) if you go with the Greek. 

Oh, but Greek has a separate form if you have two of something. It's not two criteria, it's two criterio (but long o again)! And if it's the indirect object, you give something to one criterio or to several criteriois (or criteriis if you go with the Latin). Remember this all with phenomenon and phenomena too. And then there's schema and schemata, which are a different inflectional pattern again… Maybe you should look it all up on Wiktionary. 

Incidentally, yes, octopus was taken originally from Greek, but in Greek it's oktopous, so we can get away with the Latin version. Maybe.

Übermensch, Festschrift

For most words we borrow from German, no one insists on using the German plural. But there are a few more precious words that academic types like to handle as German words and pluralize with –en or whatever. First of all, of course, you need to know that all German nouns are always capitalized. Thus, if someone is using a German noun and capitalizing it everywhere, it means they're using it as German, so they'd better get the cases right. 

The thing with German, though, is that its different noun forms for the different cases are, well, not always as different as in Latin and Greek. So. An Übermensch is a superman (in the Nietzschean mold). You can have multiple Übermenschen. Nominative, accusative, all Übermenschen. Also genitive. And dative. And I mean plural and singular. In fact, the only time you use Übermensch rather than Übermenschen is when it's the singular subject of a sentence. Give the Übermensch the other Übermensch's Übermensch? Nope. Give the Übermenschen the other Übermenschen Übermenschen.

Festschrift, which is a volume of essays celebrating some scholar (of whom you have never heard, but they think everyone knows), is dead easy. Too easy. All singulars are Festschrift, all plurals Festschriften. Remember, that includes possessives. Not the Festschrift's author. The Festschrift author. And definitely not the Festschriften's authors. Just the Festschriften authors.

karma

By this point, do you feel like you're getting karmic retribution for wanting to use the plurals from the original languages? I have bad news for you: karma is a Sanskrit word, and Sanskrit has eight different cases, in singular, dual, and plural forms, for a total of 24 inflections. Luckily, some of them use the same form — there are just 13 different forms of karma. And, since in English karma is treated as a mass object (like water and rice), you don't have to worry about the plurals. But you still can't talk about your karma's influence — it has to be your karmnah influence (and actually the n and h there stand for sounds we don't even have in English). Things happen by way of karmna, and you can be stuck in bad karmni. As you seem to be right now.

sauna

You really haven't met inflection until you've met Finnish. Finnish doesn't use prepositions much; it replaces them with a whole bunch of different inflections. Fifteen of them in singular and plural, with fun names like elative, translative, and comitative. Oh yeah.

You can make yourself really popular at the ski resort or spa when you show off this knowledge. "Do they have a sauna, or several saunat? I like to sit saunassa — oh, that means ‘in the sauna.' I guess if I could be in two of them I would be sitting saunoissa! But I'd have to come out of one of them — I'd be coming saunasta, or saunaista if I came out of more than one. Hee hee. If I came out of one and went into another, I'd be coming saunasta and going saunaan! Ha ha ha. You know, Finns just can't live saunatta — without a sauna!" You'll be the life of the party. Or maybe the death of it.

But even if you can't keep track of all that (and there's a lot more too), you'll still want to know the genitive: saunan instead of sauna's and saunojen instead of saunas' (or saunat's).

Are you just about crazy from all this? I have good news for you. As long as you stick with French, Italian, and Spanish, you're fine. They really do have just two forms for each noun: singular and plural. 

But still. If you think people will be impressed that you know octopodes, why not go all the way and use geniorum too? Look, are you trying to communicate, or to win?