Fin

/fin/

Cases

SingularPluralWhen to use it
nominativeFinthe FinnFinlethe Finnsthe plain dictionary form — the doer of the sentence
genitiveFinnithe Finn'sFinlenithe Finns'the owner — whose it is
accusativeFinnithe Finn (as the object)Finlenithe Finns (as the object)a specific, known object — whom or what
dativeFingeto the FinnFinlegeto the Finnsdirection or recipient — to where, to whom
locativeFindeat / in the FinnFinledeat / in the Finnswhere it is; also having — mende bar, "I have it"
ablativeFindenfrom the FinnFinledenfrom the Finnswhere it comes from; also "than" when comparing
When to use each case
nominative
the plain dictionary form — the doer of the sentence
genitive
the owner — whose it is
accusative
a specific, known object — whom or what
dative
direction or recipient — to where, to whom
locative
where it is; also having — mende bar, "I have it"
ablative
where it comes from; also "than" when comparing

The endings are the same ones the pronouns wear — that page explains when each case is used.

With an owner

One ownedSeveral owned
myFinimmy FinnFinlerimmy Finns
yourFiningyour FinnFinleringyour Finns
his / herFinihis / her FinnFinlerihis / her Finns
ourFinibizour FinnFinleribizour Finns
you all'sFinigizyou all's FinnFinlerigizyou all's Finns
theirFinleritheir FinnFinleritheir Finns

The overlap in the last rows is real, not a mistake: Finleri can mean "his / her Finns", "their Finn", or "their Finns" — context decides.