Case closed

A standard whine of non-Germans trying to learn German are the cases for nouns.

German has four cases (nominative, accusative, dative, genitive nominative, genitive, dative, accusative (*)). In contrast, we have 15 cases in Finnish. (But we have very few prepositions. This is why I can never get my prepositions right in English.)

(*) My colleague Herr Doktor E. points out that the cases need to be in this order. In school they're learned in this order, and for example genitive is referred to as "the second case".

The four German cases are used like this:

Nominative: the subject
Der Mann sitzt auf der Bank. (The man is sitting on the bench.)

Accusative: the direct object, and used with a fixed list of prepositions
Ich sehe den Mann. (I see the man).
Ich komme nicht ohne meinen Mann. (I won't come without my husband.)

Dative: the indirect object, and used with a fixed list of prepositions
Ich gebe dem Mann das Buch. (I give the book to the man.)
Ich wohne mit meinem Mann. (I live with my husband.)

Genitive: possession, and used with a fixed list of prepositions.
Das Buch des Mannes ist groß. (The book of the man is big.)
Ich habe nie wegen meines Mannes geweint. (I have never cried because of my husband.)

The fifteen Finnish cases are used like this:

Nominative: the subject
Mies istuu penkillä. (A/the man is sitting on a bench.)

Accusative: the object of a completed action
Söin omenan. (I ate an apple (all of it).)
Ammuin miehen. (I shot a man (and he died).)

Partitive: the object of an incomplete action
Syön omenaa. (I'm eating an apple (but I haven't eaten all of it yet).)
Ammuin miestä. (I shot a man (but he didn't die).)

Genitive: possession
Sen miehen koira on tuolla. (That man's dog is there.)

Essive: being like something, expressions related to time
Miehenä minun täytyy sanoa, että... (As a man, I have to say that...)
Kauniina päivänä on hauskaa olla ulkona. (On a beautiful day it's fun to be outside.)

Translative: transforming into something.
En voi muuttua mieheksi. (I cannot transform into a man.)

Inessive: in
talossa (in a house)

Elative: from
talosta (from a house)

Illative: into
taloon (into a house)

Adessive: on
Istun penkillä. (I'm sitting on a bench.)

Ablative: "on-from"
Nousen penkiltä (I'm getting up from a bench.)

Allative: "onto"
Istu penkille! (Sit down on a bench!)

Comitative: "with my/your/..."
Menen ulos koirineni. (I'll go outside with my dogs.)

Instructive: "with the help of something"
Viihdytin lapsia pelein ja kirjoin. (I entertained the kids with games and books.)

As a bonus, we get to combine a lot of stuff into the endings of nouns. Like this:

taloissammekin = talo - i - ssa - mme - kin = house - plular - in  - our - too = in our houses too