Learn Croatian: Smatra se da blagajnica mora biti brza. - It is believed that a cashier must be fast.

This is the impersonal use of the reflexive pronoun se. When paired with the 3rd-person singular verb smatra (from smatrati, to consider), it creates a general or passive meaning like "It is considered" or "People believe", without specifying exactly who is doing the believing.

What is the difference between smatrati and vjerovati if both can mean "to believe"?

smatrati is used for holding an opinion, expressing a stance, or considering something to be a certain way, which fits this sentence perfectly. vjerovati is used for having faith in something, trusting someone, or believing that a specific fact is true.

Why is the adjective brza in the nominative case instead of the accusative?

The verb biti (to be) does not take a direct object; instead, it acts as a bridge linking the subject to a descriptive adjective or noun. Since the subject of this clause is blagajnica (feminine, singular, nominative), the adjective brza must match it perfectly in case, gender, and number.

Could I say "mora da bude brza" instead of "mora biti brza"?

Using a modal verb directly with an infinitive (mora biti) is the standard and grammatically correct structure in Croatian. The alternative structure using "da" plus the present tense (mora da bude) is standard in Serbian, but in Croatian it sounds unnatural and should be avoided in favor of the infinitive.