Xerxian Nouns
So there are three main components to nouns: grammatical gender (aka "noun class"), grammatical number, and grammatical case.
For Xerxian, I've combined grammatical number and gender into a noun class system with 4 genders, each with a different number system. The six cases, however, apply across all the noun classes.
Noun Classes
What Are Noun Genders and Numbers?
Grammatical gender does not exclusively mean masculine/feminine. All it means is that other parts of speech (i.e. adjectives) have to change to match to the noun. Americans might be familiar with this from Romance languages like Spanish, where you would say "el techo rojo" for "red roof", but "la mesa roja" for "red table".
Grammatical number means things like singular, plural, collective, paucal, etc. Where English just has "singular" (one) and "plural" (more than one), some languages have things like "dual" (two) and "paucal" (a few/group of). There are also "mass nouns" (can't be counted without a quantifier, i.e. 'sand' vs 'grain of sand') and "collective nouns" ('flock' vs 'bird').
You can learn more about grammatical number and gender here.
Xerxian Grammatical Genders and Numbers
I wanted to have grammatical gender, but not sex-based ones. In addition, I want Xerxian to be at least somewhat influenced by alchemy. So, I created genders based on the four classical elements of alchemy: Air, Water, Earth, and Fire. Then I gave each one its own number structure, making that number structure the primary definition of the genders/noun classes.
Air - Mass, Quantified Mass
Water - Collective, Singulative, Plurative
Earth - Singular, Paucal, (Distributive) Plural
Fire - Singular, Plural
Air / Aerial Nouns - Mass, Quantified Mass
Air nouns are mass nouns - they don't have a singular or plural form because they do not exist in a countable state. For example, water: you can only count it once there is a classifier (i.e. "drops" of water) or in some kind of container ("buckets"), but there is no such thing as "two waters" (unless there is context implying a classifier, such as ordering two cups of water at a restaurant). Once there is such a classifier, though, it is now a 'quantified mass' noun.
Most elements are mass nouns, as are things like powders, gasses, and liquids. Other aerial nouns include substances; components/ingredients of another thing; certain abstract concepts, like "time"; and fields of profession or information, like "agriculture".
Water / Aquatic Nouns - Collective, Singulative, Plurative
Water nouns are collective-singulative nouns. They start out referring to a collection or group, and then the noun gets changed in some way to indicate a specific instance of it. This structure does not exist in English, but examples of this exist in Welsh and Arabic.
i.e. In the natural world, we generally see a bunch of birds together, moreso than we see one on its own. So in a collective-singulative system, "flock" might be the default term, and then you change the word to indicate "one of a flock", re: a (single) bird. This can then be turned into plural again, and in this system such a term is called a plurative. For example, if I see two birds flying around - there is clearly more than one bird, but they are not a flock. This is what the plurative form is for.
Earth / Terran Nouns - Singular, Paucal, (Distributive) Plural
Earth nouns have singular, paucal, and plural nouns. Singular is pretty obvious/self-explanatory.
Paucal nouns generally mean "a few of". In Xerxian, it often carries an additional connotation of those few being in a collection or group of some sort. There is no set number this applies to (though generally will be used for ten or fewer nouns). If you can figure out how many of the things there are just by looking at it or without having to count it off, it's probably paucal.
For the most part, the Earth plural is for anything where there is too many to figure out many there are just by glance. But you might also use it for a few of something, if they are not "together" so to speak. So if you have five tools on your belt, you'll use the paucal, and if you've got a large bag of tools you'll use the plural. But if five workmen are each holding up their own tool, you might also use the plural then, even though there are only five tools.
This noun class is common in day to day life, as it usually refers to most man-made objects, as well as people and some animals.
Fire / Incendiary Nouns - Singular, Plural
Fire nouns follow the pattern most familiar to English speakers, singular and plural, so it doesn't need much explaining.
This noun class encompasses anything with a beginning and an ending. The most obvious examples are weather events and natural phenomena, such as rain, earth quakes, day and night, sunrise and sunset, etc. This also includes events like lessons, ceremonies, or festivals abstract ideas like "life", and common temporally spanning occurrences like "stories". And, of course, fire itself - fires have to be started, and eventually they all burn out.
"Multi-Gender"/Multi-Class Nouns
In gendered languages, there are often nouns which can take on more than one gender, and usually change nuance or meaning when they do, such as in Spanish. For example, the world "busca" means a 'pager' in the masculine form, but a 'search' in the feminine form.
Similarly, some nouns in Xerxian can taken on multiple genders, and usually change nuance or meaning when they do.
For example, "alchemy" is normally an Air noun - alchemy is a general activity/abstract concept, so therefore it is effectively a mass noun. When used as a Fire noun, it means a transmutation. Where in ~~Amestrian~~ English, there are two separate words for these two things ("alchemy" vs "transmutation"), in Xerxian, there is just one word, but with different genders/classes to mean the two different things.
Noun Cases
What are Noun cases?
Grammatical case is the role of the noun in a sentence. For example, in the sentence "Mei kisses Al", Mei is the subject, Al is the object. English uses word order to indicate case, so "Mei kisses Al" and "Al kisses Mei" have different meanings. Some languages change (decline) their nouns to indicate case. The only example of this in English is in pronouns: "she kisses him" vs "he kisses her". In a language where all the nouns do this (and has free word order), "she kisses him" and "him kisses she" mean the same thing: that Mei is the one kissing, and Al is receiving the kiss.
The subject does not always have to be "committing" the verb, though. In the sentence, "Your face asplode!", the subject "your face" is carrying out the verb of "exploding", even though in context this verb is actually being caused by an external agent, not the subject.
There are also a lot of other noun 'roles', such as the instrumental case. In the sentence, "Winry hits Ed with a wrench", Winry is the subject, Ed is the object, and the wrench is the 'instrument' of that sentence's verb.
You can learn more about grammatical case here.
Xerxian Cases
Mesopatamian languages (being one of the main inspirations for Xerxian) tended to have a lot of cases. However, the majority of Xerxian's other inspiration languages (i.e. Semitic, Ancient Greek) used something called nominative-accusative, which is different from Sumerian's ergative-absolutive system. So, I settled on these cases (with quick demonstration/simple explanation):
Alphonse sent the letter to Edward's house in Risembool via courier.
Nominative (subject) = Alphonse - he's the one doing the sending
Accusative (direct object) = the letter - the thing that's experiencing the verb/being sent
Dative (indirect object) = house - the thing having something sent to it
Genitive (possessive) = Edward - who the house belongs to
Locative (proximity/location) = in Risembool - where the sending is happening
Instrumental (tool/method) = via courier - how the verb/sending is happening
This is also the most common noun/word order they appear in, in Xerxian (though not the only one). You can find more information about these cases on Wikipedia.