Nouns have an impressive number of cases in Finnish, and this is often seen as a major difficulty in learning the language. However, cases largely correspond to prepositions in English, and learning a case is not very different from learning a preposition.
Admittedly, case inflection has some complications. Learning which case should be used in each context is comparable to learning to select the right preposition in English. Just like we say “interested in…” in English, using specifically the preposition “in”, the corresponding Finnish word kiinnostunut requires a specific case—the elative, with suffix sta or stä—for the word expressing the target of the interest. This does not mean that the elative always corresponds to “in”; in fact, it much more often corresponds to “from” or “about”.
Most cases in Finnish have mixed usage: they may have a specific concrete meaning, but they are also used in many other meanings or simply as part of a phrase. English uses phrases like “I fell in love with her” so that we may ask why we use “with” here—after all, love can be one-sided. Similarly Finnish uses phrases like Rakastuin häneen, and there is no logical reason for using the illative case (häne|en) here; it’s just part of the phrase.
Since some verbs or other expressions often require a specific case for the associated noun, all general descriptions of the meanings of cases are unavoidably incomplete. For example, the concrete meaning of the elative is locational: it corresponds to the English word “from”; e.g. Suomesta, the elative of Suomi (Finland), means “from Finland”. However, the elative is often used in other meanings, and one of them was mentioned above, e.g. Olen kiinnostunut Suomesta. This has no concrete locational meaning, any more than “in” has in the corresponding English sentence “I am interested in Finland”.
The word rection is sometimes used to refer to a phenomenon where a verb “requires” a particular case (or, in other languages, preposition) to be used for an associated noun.
Much of the use of cases is described in dictionaries in descriptions and examples of using verbs. For example, an entry for the verb kiinnostua (to become interested in) describes how a noun in the elative is used with it. Similarly, if you look up the word rakastua (to fall in love), you should see that it is used with a word in the illative case, so that you can form a sentence like Rakastuin Maijaan (I fell in love with Maija), using the illative form Maijaan. Thus, there is usually no explicit statement about rection, but the existence of rection can mostly be inferred from examples.
The cases in Finnish can be divided in three groups on structural grounds:
Three of the grammatical cases have some specialized locational use, as a remainder of their origin: essive (-nA case), partitive (-A or -tA case), and translative (-ksi case).
The cases have rather different frequencies of use:
Language learners certainly have difficulties with Finnish cases, but the difficulties have often been exaggerated. In order to read very simple texts in Finnish, like menus of restaurants, texts in product packages, and proper names, it mostly suffices to recognize just three common cases: the nominative (no suffix in the singular), the partitive (suffix a, ä, ta, or tä) and the genitive (suffix n). Locational cases are rare in such contexts, except for the inessive (suffixes ssa and ssä) and the adessive (suffixes lla and llä).