kostüm

/kostym/

Cases

SingularPluralWhen to use it
nominativekostümthe suitkostümlethe suitsthe plain dictionary form — the doer of the sentence
genitivekostümnüthe suit'skostümlenithe suits'the owner — whose it is
accusativekostümnüthe suit (as the object)kostümlenithe suits (as the object)a specific, known object — whom or what
dativekostümgeto the suitkostümlegeto the suitsdirection or recipient — to where, to whom
locativekostümdeat / in the suitkostümledeat / in the suitswhere it is; also having — mende bar, "I have it"
ablativekostümdenfrom the suitkostümledenfrom the suitswhere 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
mykostümümmy suitkostümlerimmy suits
yourkostümüngyour suitkostümleringyour suits
his / herkostümühis / her suitkostümlerihis / her suits
ourkostümübüzour suitkostümleribizour suits
you all'skostümügüzyou all's suitkostümlerigizyou all's suits
theirkostümleritheir suitkostümleritheir suits

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