Welcome to the first declension of Aril. These are most words that end in vowels (unless the word has a stressed i sound, in which case you want the third declension, or is a general or abstract noun, in which case you want the fifth declension.)
Brief Explanation of Cases
You probably know this already, but a 'case' simply refers to a change made in a word (usually a noun) to give it a particular grammatical function (as indirect object, direct object, etc.) or illustrate a particular physical relationship between the noun and something else (in English we usually do this with prepositions). At least, I think that's the definition. As I may have said before, I'm not a linguist, and while I think I know how to use cases, I'm not sure I know how to define them.
The first declension, like the second declension and the fifth declension, mainly only uses four cases:
Nominative- The case of the subject;
Genitive-Case illustrating possession;
Accusative-Case indicating direct object;
and Dative-Case indicating indirect object.
(If you're interested in more technical definitions of the cases, try entering terms like "accusative case" in any good search engine; it should give you some good hits).
Here are the forms the cases take for the first declension, working with the nouns fanta 'jewel;' nanda 'world;' and enhelo 'drink;'
Nom. fanta, jewel fantai, jewels
Gen. fantan, jewel's fantani, jewels'
Acc. fantas, jewel fantasi, jewels
Dat. fantad, to the jewel fantaidi, to the jewels
Thus, the genitives fantan and fantani mean "jewel, of the jewel" and "jewels', of the jewels" respectively. Genitives are often required in Aril as adjectives, where English would simply use another noun. For example, a possible phrase in English is the jewel road; everyone knows what it means, and there is no need to spell it out further. But the phrase in Aril would need to be emerís fantani, 'the road of jewels;' or, if a road made of jewels were the noun, emerís fanté. é is a common adjectival ending, especially for nouns of the first declension.
The accusatives, fantas and fantasi, are required for any sentence in which the jewels are the direct object rather than the subject. For example, if one were to say Elfyr fantai, "The jewels shone," then only the nominative form is needed. But if one were to say Yolran fantasi, "I don't own any jewels," the accusative form is needed; the jewels are something possessed in this sentence, the object of the verb yol, 'to possess or own.' This is another distinction that English tends to obscure, largely because it takes care of the problem with word order: objects almost always come after verbs, and subjects before them.
The datives, fantad and fantaidi, are a little easier, if only because English, as well as Aril, requires special words for indirect objects. The datives in this case mean "to/for the jewel" and "to/for the jewels" respectively. In the sentence Azuelzhan fantaidi cal, "I will serve you for the sake of the jewels," the dative is required, not least because of the direct object cal, 'you.' This sentence might mean either that the person speaking is serving because of certain jewels, or becauuse she is being paid in jewels. It would not be clear without more elaboration; nontheless, the dative is required.
Most other relationships are expressed by suffixes that do not vary from form to form, and are simply added to the nominative form, if the noun is the subject, or to the accusative form, if the noun is the object--like so:
nandedi 'in the world'--nanda 'world + -edi, a common ending meaning 'within.' Before suffixes beginning with a vowel, if the form in question ends with a vowel, the noun will usually drop its vowel.
enhelodum 'without a drink'--enhelo 'drink or draught' + -dum 'without,' '-less.' This transforms the noun into an adjective.
