Learn Croatian: Moramo vidjeti svaku izložbu. - We must see every exhibition.

The word svaki acts like an adjective, meaning it has to match the gender, number, and case of the noun it describes. Because it describes izložbu (a feminine, singular noun in the accusative case), it takes the matching feminine accusative ending, becoming svaku.

Why does the word for exhibition end in -u (izložbu) instead of -a (izložba)?

The exhibition is the direct object of the sentence, meaning it is the thing being seen. Direct objects need to be in the accusative case. For feminine nouns that normally end in -a like izložba, the accusative ending changes to -u.

Why doesn't the verb vidjeti change its ending to match 'we'?

vidjeti is in its infinitive (dictionary) form. It does not change because it follows the modal verb moramo (we must). Just like in English where we say 'must see' instead of 'must seeing', the second verb in this pair stays in the infinitive.

Where is the word for 'we' in this sentence?

You could say mi moramo, but the pronoun is usually left out in Croatian. The -mo ending on the verb moramo already makes it perfectly clear that 'we' are the ones who must do the action.