The accusative case: -ı / -i / -u / -ü
Turkish marks direct objects only when they're definite — meaning 'the X' rather than 'a X'. The marker is -ı, -i, -u, -ü (chosen by harmony) attached to the noun. Compare: Kitap okuyorum = I'm reading a book / I read books (general); Kitabı okuyorum = I'm reading the book (a specific, known one). After a vowel a buffer y appears: arabayı (the car). The accusative is the most common case you'll meet, and getting comfortable with the definite/indefinite split is the main hurdle for English speakers.
When to use it
Use the accusative when the object is specific or known: a noun introduced earlier, a noun with bu (this) or şu (that), a proper name, a noun with a possessive suffix. Bu kitabı okuyorum = I'm reading this book. Ali'yi gördüm = I saw Ali. Annemi seviyorum = I love my mother. Skip the suffix when the object is generic or part of the verb: kitap okumak = to read (books in general / book-reading), çay içmek = to drink tea, yemek yapmak = to cook (literally 'do food'). Many such verb+noun combinations function almost as compound verbs.
The four-way harmony
Last vowel of the noun decides the suffix vowel: after a, ı use -ı; after e, i use -i; after o, u use -u; after ö, ü use -ü. kitap → kitabı (after a, plus the p→b softening). ev → evi. okul → okulu (school). göz → gözü (eye). After a vowel, slip a buffer y in: araba → arabayı, kapı → kapıyı (the door), kedi → kediyi (the cat).
Personal pronouns
The pronouns have their own accusative forms — mostly regular, with one quirk: ben → beni (me), sen → seni (you), o → onu (him/her/it), biz → bizi (us), siz → sizi (you all), onlar → onları (them). Beni seviyor mu? = Does he/she love me? Onu görmedim = I didn't see him.
Definite indefinite — the trap
English doesn't always make the definiteness obvious. Check yourself: 'I'm reading a book' (any book / book-reading) → Kitap okuyorum. 'I'm reading the book' (the one we discussed) → Kitabı okuyorum. 'Are you watching TV?' is generic → Televizyon izliyor musun? But 'Are you watching the news?' is specific → Haberleri izliyor musun? Proper nouns are always definite: Ayşe'yi gördüm (I saw Ayşe), with apostrophe before the suffix. Anything with a possessive suffix is also definite: kitabımı okuyorum = I'm reading my book.
Stacking with possessive
When a possessed noun is also a definite object, the possessive suffix comes first, then the accusative. kitap → kitabım (my book) → kitabımı (my book, accusative): Kitabımı okuyorum = I'm reading my book. arabası (his car) → arabasını (with buffer n that appears specifically after the 3rd person possessive -ı / -sı): Onun arabasını sevmiyorum = I don't like his car. The buffer n after -(s)i is a quirk of the 3rd person — for now just memorize the cluster -sını / -sini / -sunu / -sünü.
