Learn Finnish: Kääntäjä kääntää kirjan ruotsiksi. - The translator translates the book into Swedish.

The ending -ksi is the Translative case. While it often shows a change of state, it is also the standard case used for languages of communication. Whenever you express translating, writing, or speaking in or into a specific language, you use the translative. Here, ruotsiksi means into Swedish.

Why is the object kirjan instead of kirjaa?

Kirjan is the total object, marked with the -n ending. This implies the translator translates the entire book from start to finish. If they were in the middle of translating it right now, or only translating a part of it, you would use the partitive case (kirjaa).

Are kääntäjä and kääntää related?

Yes! You can add the suffix -jä (or -ja, depending on vowel harmony) to a verb to describe the person who does that action. It works just like the -er ending in English. So kääntää (to translate) becomes kääntäjä (translator).

Doesn't the verb kääntää also mean to turn?

That is correct. Kääntää is a versatile verb used both for physically turning something (like a car or a page) and for translating text. The context of a book and a language case (ruotsiksi) makes it completely clear that it means translates here.