Learn Croatian: Ne smijete hraniti medvjede, to je zabranjeno. - You must not feed the bears, that is forbidden.

The verb smjeti means "to be allowed to," so ne smijete literally means "you are not allowed to." In English, we often use "must not" for rules and prohibitions, which is exactly how ne smjeti is used in Croatian. Be careful not to confuse this with a lack of obligation: "you don't have to" would be ne morate.

Why is the word for bears medvjede instead of medvjedi?

Medvjede is in the accusative case because the bears are the direct object receiving the action of being fed. For regular masculine nouns like medvjed, the plural nominative (the subject) ends in -i (medvjedi), but the plural accusative (the object) ends in -e (medvjede).

Why does zabranjeno end in -o?

Zabranjeno is a passive adjective meaning "forbidden." It ends in -o because it must agree in gender with the word it refers back to. Here, it refers to the general neuter pronoun to (that). If it referred to a feminine noun, it would be zabranjena.

Could I just say zabranjeno je instead of to je zabranjeno?

Yes, you could say zabranjeno je (it is forbidden) as a general impersonal statement. However, adding to (that) specifically points back to the entire previous action of feeding the bears, making the connection slightly stronger, just like saying "that is forbidden" versus "it is forbidden" in English.