The genitive case: -ın / -in / -un / -ün
Where English uses an apostrophe-s ('Ali's car') or 'of', Turkish marks the owner with the genitive -(n)ın / -(n)in / -(n)un / -(n)ün. Crucially, this never travels alone — the thing being owned ALSO gets a possessive suffix, agreeing with the owner. Ali'nin arabası = Ali's car (literally 'Ali-of his-car'). Annemin evi = my mother's house. After a vowel a buffer n appears (the -nin form). The pair of suffixes — genitive on the owner, possessive on the possessed — is one of Turkish's signature constructions.
Building the chain
Step 1: put the genitive on the owner. Ali ends in a vowel → Ali'nin (with apostrophe because it's a proper name). araba ends in vowel → arabanın = of the car. ev ends in consonant → evin = of the house. Step 2: put the 3rd person possessive -(s)i on the thing owned. araba → arabası. ev → evi. Combine: Ali'nin arabası = Ali's car. annemin evi = my mother's house (here annem itself already has 1st-person possessive, so the genitive attaches to that: annem-in).
Vowel harmony for genitive
Four-way harmony like the accusative: after a, ı → -ın; after e, i → -in; after o, u → -un; after ö, ü → -ün. kitap → kitabın (and the p→b softening: 'of the book'). ev → evin. okul → okulun. göz → gözün. After a vowel, the buffer n turns the suffix into -nın / -nin / -nun / -nün: araba → arabanın, kedi → kedinin, kapı → kapının.
Proper names get the apostrophe
All suffixes attach to proper names with an apostrophe: Ali'nin (Ali's), Ayşe'nin, İstanbul'un (Istanbul's), Türkiye'nin (Turkey's). This makes the boundary visible since the name is capitalized. With everyday nouns, no apostrophe: kitabın (of the book) flows as one word. Ali'nin kitabı = Ali's book. İstanbul'un nüfusu = Istanbul's population.
The pronoun chain
You met these in lesson 11 as 'genitive pronouns' — they're literally the pronoun + genitive: benim (of me / my), senin (of you), onun (of him/her), bizim (of us), sizin (of you pl), onların (of them). Pair each with the appropriate possessive suffix on the noun: benim kitabım (my book), senin kitabın, onun kitabı, bizim kitabımız, sizin kitabınız, onların kitabı / kitapları. The pronoun is optional; the suffix on the noun is required.
When NOT to use the genitive
Compound nouns describe a type, not a possession, and only the second noun gets the -(s)i suffix — no genitive on the first. Türk kahvesi = Turkish coffee (the kind), not 'a Turk's coffee' which would be Türk'ün kahvesi. otobüs durağı = bus stop. havayolu = airline (literally 'air-road'). ders kitabı = textbook. The pattern 'X-of-Y' in Turkish covers possession, but bare 'X Y-suffix' covers categorization. With a definite owner you almost always want both suffixes.
