nužno

1. (often with negation) necessarily; inevitably
   To ne znači nužno da je on kriv. — That doesn't necessarily mean he's guilty.
   Skuplje nije nužno i bolje. — More expensive isn't necessarily better.
2. (nužno je + inf./da) it is necessary
   Nužno je da odmah pozoveš lekara. — You need to call a doctor right away.
   Da li je baš nužno da idemo danas? — Is it really necessary for us to go today?

In the sense "necessary" it usually appears as nužno je da… or nužno je + infinitive.