Genitiv.
Whose is it?

The Genitiv shows belonging. It answers one question → Wessen? "Whose?" It's the fourth case, the one that marks ownership, and the one where the noun itself changes in addition to the article.

Also: This completes the four-case series. Start from Nominativ, Akkusativ, and Dativ if you haven't yet.

The question that finds the owner

Ask Wessen? "Whose?" Whatever answers that question is Genitiv. It modifies another noun by showing it belongs to, or comes from, something.

• Das istdasAuto desMannes. Whose car? Des Mannes. → Genitiv.
• Das istdieTasche derFrau. Whose bag? Der Frau. → Genitiv.
• Das istdasSpielzeug desKindes. Whose toy? Des Kindes. → Genitiv.

The Genitiv noun always follows the thing it belongs to. The order is: thing → owner in Genitiv.

What Genitiv does to articles

Two things happen at once. The article changes, and most masculine and neuter nouns pick up an -s or -es ending on the noun itself. Feminine and plural don't add that noun ending, but the article still changes.

One group of masculine nouns breaks this: the weak nouns, which take -n or -en instead of -s in every case except the Nominativ. desStudenten, desJungen, desHerrn, desNachbarn, desMenschen: never des Studentes. Most of them name people or animals and end in -e in the Nominativ.

Gender	Nominativ	Genitiv	Noun ending
Masculine	derMann	des Mannes	+es
Neuter	dasKind	des Kindes	+es
Feminine	dieFrau	derFrau	none
Plural	dieKinder	derKinder	none

The two new article forms to learn → des (masculine and neuter) and der (feminine and plural). Feminine Genitiv der looks identical to masculine Nominativ der. Context tells them apart.

The -s on the noun

Short nouns (one syllable) usually take -es: des Mannes, des Kindes. Longer nouns usually take just -s: des Lehrers, des Computers. Both are correct. In everyday writing you'll see both, and both are understood immediately.

Indefinite articles in Genitiv

Gender	Nominativ	Genitiv
Masculine	einMann	einesMannes
Neuter	einKind	einesKindes
Feminine	eineFrau	einerFrau

Where Genitiv actually appears

Genitiv is less common in everyday spoken German than the other three cases. It shows up most reliably in three places.

Written and formal German. News articles, academic texts, contracts, official documents. "DieEntscheidung desGerichts." "DerName desUnternehmens."

Fixed Genitiv prepositions. A group of prepositions always takes Genitiv → wegen, trotz, während, statt, aufgrund, bezüglich. You see the preposition, you know the case. See These prepositions only speak Genitiv for the full list.

Names and proper nouns. "Das ist Annas Buch." German adds -s directly to names, just like English, but without the apostrophe, unless the name already ends in an s sound (s, ss, ß, tz, z, x). Then it takes an apostrophe instead: "Max' Fahrrad," not "Maxs Fahrrad."

von as a spoken substitute

In spoken German, Genitiv is often replaced by von + Dativ. "DasAuto desMannes" becomes "DasAuto vondemMann" in casual speech. Both mean the same thing. In writing, the Genitiv form is expected. In conversation, either works.