Altıncı kitabı seçtim.

Because Turkish normally uses subject–object–verb order.

So in Altıncı kitabı seçtim, the object altıncı kitabı comes before the verb seçtim.

A very literal English-style order would be:

• I = hidden in the verb
• the sixth book = altıncı kitabı
• chose = seçtim

So Turkish puts the verb last by default.

Seçtim can be broken into:

• seç- = choose, select
• -ti = past tense
• -m = I

So seçtim means I chose or I selected.

The dictionary form of the verb is seçmek.

In Turkish, the verb ending often already shows the subject.

Here, -m in seçtim tells you the subject is I. Because of that, ben is not necessary.

You could say Ben altıncı kitabı seçtim, but that usually adds emphasis, like I chose the sixth book.

Because the noun is a definite direct object.

In Turkish, when the direct object is specific or definite, it usually takes the accusative ending. That is what you see here.

• kitap = book
• kitabı = the book, as a specific object

So Altıncı kitabı seçtim means you chose a particular book: the sixth book.

If you said kitap without that ending, the meaning would be less definite or less natural in this context.

Turkish does not have a separate word exactly like English the.

Instead, definiteness is often shown in other ways. In this sentence, one important clue is the accusative ending on kitabı, which helps show that it is a specific book.

So Turkish does not translate English word-for-word here. The idea of the sixth book is conveyed without a separate article.

Because of a common sound change in Turkish called consonant softening.

Some nouns ending in p, ç, t, k often change that final consonant when a vowel-initial suffix is added.

Here:

• kitap ends in p
• the accusative suffix starts with a vowel
• so p becomes b

That gives:

• kitap + ı → kitabı

This is very common in Turkish.

Here it is accusative, not possessive.

That is a very common question, because kitabı can sometimes also look like his/her book in other contexts. But in this sentence, it means the book as the object of the verb.

A useful comparison:

• kitabı = the book, as a definite object
• kitabı = his/her book, in some other contexts
• kitabını = his/her book, as a definite object

So if the meaning were I chose his/her sixth book, Turkish would normally need something like Altıncı kitabını seçtim or Onun altıncı kitabını seçtim.

Altıncı is the ordinal form of altı, which means six.

Turkish makes ordinals with a suffix that appears in different vowel-harmony forms, often shown as -(I)ncI.

So:

• altı = six
• altıncı = sixth

Other examples:

• bir → birinci = first
• iki → ikinci = second
• üç → üçüncü = third

So altıncı kitap means sixth book.

Not if you want the normal meaning I chose the sixth book.

Without the accusative ending, kitap sounds non-specific or indefinite. In many contexts, that would not express the same idea.

So:

• Altıncı kitabı seçtim = I chose the sixth book
• Altıncı kitap seçtim = not the normal way to say that

If you really wanted an indefinite idea, Turkish would usually express it more clearly in another way, often with bir.

Yes, Turkish word order is flexible, but changing it changes the focus or emphasis.

The neutral order is:

• Altıncı kitabı seçtim

You might also hear:

• Seçtim altıncı kitabı

but that is more marked and would sound unusual in many ordinary contexts.

For learners, the best default is to keep the standard order:

• object before verb
• verb at the end

So Altıncı kitabı seçtim is the most natural basic version.