Learn Croatian: Ne smijemo hraniti patke starim kruhom. - We must not feed ducks with old bread.

The verb smjeti means "to be allowed to." When negated as ne smijemo, it literally means "we are not allowed to," which equates to "we must not" in English. If you want to express "we don't have to" (meaning there is a lack of obligation), you would negate the verb morati (must) to get ne moramo.

The English translation says "with old bread", so why is there no preposition like "s" or "sa" before "starim kruhom"?

In Croatian, you use the instrumental case without a preposition when a noun is the means, tool, or "instrument" used to perform an action. Since the old bread is the material being used to feed the ducks, it takes the bare instrumental. You only use s or sa when talking about company or association, such as walking with a friend.

Why doesn't the adjective ending match the noun ending here (starim vs. kruhom)?

Adjectives and nouns have different declension patterns. For masculine nouns in the instrumental case, the ending is usually -om or -em. However, masculine adjectives in the instrumental case take the -im ending. Therefore, stari becomes starim to match the masculine noun kruhom.

Why does "kruh" take the "-om" ending rather than "-em" in the instrumental case?