Learn Croatian: Napiši kratak sažetak knjige. - Write a short summary of the book.

Napiši is the imperative form of the perfective verb napisati. You use the perfective aspect here because you are asking someone to complete a single, specific task (writing one summary from start to finish). If you used the imperfective piši, it would mean "be writing" or emphasize the ongoing process.

Why is the adjective kratak used instead of kratki?

We use the indefinite form kratak because it translates to "a short summary" rather than "the short summary". For masculine inanimate nouns in the accusative case like sažetak, the indefinite adjective form matches the nominative (kratak), while the definite form would be kratki.

Is sažetak in the nominative or accusative case here?

It is in the accusative case because it is the direct object of the verb napiši. However, because sažetak is a masculine inanimate noun, its accusative form looks exactly the same as its nominative form.

Why is knjige in the genitive case?

The genitive case is used here to show possession or association, translating exactly to the English "of the book". When one noun describes or belongs to another noun, the second one takes the genitive.