Each sentence is grammatical except for one word in the wrong case, highlighted in yellow. Choose the form that fixes it.
Ask what governs the highlighted word: a verb (čekati + accusative, pomoći + dative, bojati se + genitive), a preposition (iz + genitive, sa + instrumental, o + locative), or direction vs location after u/na.
Then check the noun itself: animate masculine accusative copies the genitive (vidim psa), and some nouns have extended stems (vreme → vremena).