Learn Croatian: Gdje je knjiga? - Where is the book?

In Croatian, you can usually figure out a noun's gender just by looking at its last letter. The word knjiga ends in -a. This is the standard ending for feminine nouns in their base dictionary form, which is called the nominative case.

Why is knjiga in the nominative case in this sentence?

The nominative case is used for the subject of a sentence, which is the main person or thing the sentence is about. Since we are asking about the location of the book, the book is the subject. Therefore, knjiga stays in its basic dictionary form.

Why does je come right after Gdje?

The word je (meaning 'is') is a short, unstressed word called a clitic. A major rule in Croatian is that clitics must take the second position in a sentence. Here, the question word Gdje takes the first spot, so je naturally slides into the second spot.