Learn Croatian: Tražit ćemo pravu stvar umjesto zamjene. - We will look for the real thing instead of a replacement.

Because it follows the preposition umjesto (instead of). This preposition always requires the following noun to be in the genitive case. Since zamjena (replacement) is a regular feminine noun ending in -a, its genitive singular form ends in -e.

Why does the adjective pravu end in -u while the noun stvar doesn't seem to have a case ending?

The phrase pravu stvar is the direct object of the sentence, so it must be in the accusative case. Stvar is a feminine noun that ends in a consonant. For this specific group of nouns, the accusative form is identical to the nominative form. However, the adjective describing it (prava) still follows the regular feminine declension rules, so it changes to pravu.

Why is the verb written as tražit instead of its full infinitive form tražiti?

This is a standard spelling and pronunciation rule for the Croatian future tense. When an infinitive verb ending in -ti comes directly before the future auxiliary verb (like ćemo), the final -i is dropped. So, tražiti becomes tražit.

Does the verb tražiti mean to look for or to ask for?