You may have never heard of declension, or inflected languages, but it’s something you use every day: to distinguish one dog from two dogs, you add an “s” to the end of the noun in its nominative (that is, dictionary) form. Modern English, however, is a very weakly inflected language, though its badass great-grandfather, Old English, was moderately inflected; this same decline of declension happened over many centuries to other Indo-European languages, like German and Danish. If you’ve ever taken Latin, you can probably understand the evolutionary tendency away from strong inflection: it was inflected in about in a million ways in gender, tense, and case.
Most Slavic languages, however, retained their strong inflection (Czech even managed somehow to become more complicated), a fun fact in the fine print of studying Russian. (It’s one of those things that language teachers try, for a while, to conceal from you, so as not to cause brain implosion, only to spring it on you halfway through the first semester with a guilt-laden “Surprise!”) I, however, being freakishly possessed of a passion for abstract puzzles, of which foreign grammar exercises comprise a sizable chunk, was enchanted. I had studied French before, and was used to the stricter-than-English “inflection” (although no one uses that word in teaching high school Romance languages) of French’s conjugations and tensions and the need to synchronize adjectival endings with noun gender. Russian kicks it up a notch: the endings of adjectives and nouns must be declined in one of six cases, depending on the grammatical role of the words in the sentence at hand. (Oh, and there are three genders, but they actually relate to word endings’ spelling instead of being arbitrarily assigned, plus there are no articles – cough, cough, French.)
The first case is the aforementioned nominative – a word in its pure, dictionary form, and the form of a word used when it’s acting as the subject of a sentence. “The pen is red,” in Russian: “Ruchka krasnaya.” Next is genitive, an ending that signifies possession, so when added to a noun, it’s like inserting “of” before the noun. (English actually does this with the “ ’s ” ending! – the pen’s ink is red, or “Chernila ruchki krasnaya.”) Then, dative, which indicates that something else is being done to or directed at that noun. Imagine that you could say “I gave the pen a rose” (um, suspend your disbelief for a second) – “Ya ruchke rozu podarila”. Accusative is used when the noun at hand is acting as a direct object (“I bought a pen” is “Ya kupila ruchku.”). Instrumental indicates that the noun is acting as, well, an instrument (“I write with a pen”: “Ya ruchkoy pishu”). Finally, prepositional is used with, well, prepositions, and indicates that things are happening in that town, on that platform, at the bottom of the ocean, or wherever (“A fly walked over the pen,” “Mukha polzaet po ruchke.”).
Okay, your eyes are glazing over, I know. Everyone may not derive the same sense of gratification and serenity from copying out noun forms as I do, but the really cool part about the case system is that it allows for flexible word order. Whereas English word arrangement conveys meaning (“I gave the girl a rose” is not the same as “I gave the rose a girl”), Russian word order can be quite flexibly rearranged thanks to the preciseness of case endings, whereby “rose” would look different from “girl,” and that difference would explain who is doing what to whom. This means that Russian poetry is especially rich (and untranslatable); moreover, the harmony of the various endings across adjectives and nouns of the same genders means that rhyme is quite natural and thus extremely prominent (more untranslatability). Cases are often frustrating, but after a few months of studying a language using them heavily, you, too, soon bemoan will of precision English grammar of the lack.
For more on this topic, check out more articles about the difficulties of translation.