The instrumental case is the case of companionship and instrumentality.

example

  1. Hrvoje ide u školu sa svojim prijateljima. (Hrvoje goes to school with his friends.
  2. Iskopala sam krumpir rukama. (I’ve dug out the potatoe with my hands. )

  1. Hrvoje goes to school with his friends meaning they are accompanying him.
  2. The subject (a woman) dug out a potatoe with her hands. Her hands are the instrument of the action, so the intrumental case is used. Note that there is no preposition before the noun in the instrumental case. When a noun is in the intrumental case and it is the instrument of the action, preposition is not necessary.

Prepositions that usually preceed a noun in the instrumental state: s(a), pred, za, nad(a), pod(a), među.