cases
In Finnish cases indicate the same meaning English indicates by prepositions and word order. Cases refer to an inflection of nouns, adjectives, pronouns, numerals, infinitives, participles, and adverbs. There are 15 cases, of which only 12 are fully functional.
The cases are formed by adding endings, i.e., suffixes to the word stem, possibly changing some vowels or consonants in the process to make the word easier to pronounce. The most important of these stem changes are consonant gradation and vowel change i ~ e. In short, case suffixes are not difficult but the hard part is to know if you should change the word stem and how to do that.
As a result of adding endings to the word stems Finnish words tend to be long, but there are fewer of them in a sentence than in English. Also, despite the large number of cases, Finnish is actually a lot simpler than German or Russian in this respect. Both Russian (six cases) and German (four cases) utilize three grammatical genders, each with its own set of endings, compounding the amount of memorization. Also, adjectives may use different endings than the nouns they modify, exponentially confusing things. So there!