Learn Croatian: Psi vole jesti kosti. - Dogs like to eat bones.

The direct object of jesti (to eat) must be in the accusative case. The singular word for bone is kost. Because it is a feminine noun that ends in a consonant, it belongs to the special feminine i-declension. For these nouns, the accusative plural ending is simply -i, giving us kosti.

Is kosti also the nominative plural form?

Yes! A handy feature of feminine i-declension nouns is that their nominative plural (used for subjects) and accusative plural (used for objects, as in this sentence) are completely identical. If you wanted to say 'The bones are big', you would also use kosti as the subject.

The singular word for dog is pas. Why did the 'a' disappear in the plural form psi?

The noun pas contains what is known as a 'fleeting a'. This is a vowel in the final syllable of the base word that drops out as soon as a grammatical suffix is added. When we attach the standard masculine plural ending -i, the 'a' vanishes, shrinking the word down to psi.

Could I say Psi vole da jedu kosti instead of using the infinitive jesti?