У книзі шість сторінок.

Because у here means in, and after this preposition Ukrainian uses the locative case when talking about location.

• книга = book (dictionary form, nominative)
• у книзі = in the book (locative)

So книзі is the locative singular form of книга.

Here у means in.

In many cases, у and в both mean in and are interchangeable in meaning. The choice is often made for easier pronunciation and smoother sound.

So:

• У книзі шість сторінок.
• В книзі шість сторінок.

Both are possible, though one may sound more natural depending on the surrounding sounds.

Ukrainian often omits the present-tense verb to be.

So where English says:

• There are six pages in the book

Ukrainian can simply say:

• У книзі шість сторінок.

There is no need to add a present-tense є in a basic sentence like this. The meaning is understood automatically.

This is because numerals affect the form of the noun.

After п’ять and higher numbers, Ukrainian usually uses the genitive plural form of the noun.

• сторінка = page
• сторінки = pages / also some other forms depending on context
• сторінок = genitive plural

Since шість is six, the correct form is:

• шість сторінок

This is a very important number rule in Ukrainian.

Сторінок is the genitive plural form of сторінка.

You see this form because it follows the numeral шість. In Ukrainian, numbers from 5 upward normally require the noun in the genitive plural.

So:

• одна сторінка
• дві / три / чотири сторінки
• п’ять / шість / сім сторінок

Yes. Ukrainian word order is more flexible than English word order.

У книзі шість сторінок is natural and neutral. But other orders may also be possible depending on emphasis, for example:

• Шість сторінок у книзі.

This might emphasize six pages more strongly.

Even though word order can change, the original sentence is a very normal way to say it.

Yes, the meaning is very similar.

• У книзі шість сторінок. = literally In the book, six pages
• Книга має шість сторінок. = The book has six pages

The first version is a very natural Ukrainian way to express existence or quantity in something. The second uses the verb має (has), which is also correct.

A simple approximate pronunciation:

• книзі ≈ KNYEE-zi
• сторінок ≈ sto-ree-NOK

A few tips:

• кн at the start of книзі may feel unusual for English speakers.
• The і sounds like ee in see.
• In сторінок, the stress is on the last syllable: -нок.

Because Ukrainian has no articles like English a/an/the.

So книга can mean:

• a book
• the book

The exact meaning depends on context. In У книзі шість сторінок, English often translates it as in the book, but Ukrainian does not need a separate word for the.