rijeku

In a sentence

• Šetali smo uz rijeku cijelo poslijepodne.

  We walked along the river all afternoon.

  "Rijeku" here is in the accusative case, indicating motion along the river, as part of the activity.

• Preplivali smo rijeku prije zalaska sunca.

  We swam across the river before sunset.

  "Rijeku" is used in the accusative case, denoting the object of the action of swimming.

• Djeca su promatrala ribe koje plivaju u rijeci.

  The children were watching fish swimming in the river.

  Here "rijeci" is in the locative case, indicating the location where the fish are swimming.