Questions & Answers about Za putovanje su potrebne putovnica i karta.
Because the sentence is talking about two things: putovnica and karta.
- je = is (singular)
- su = are (plural)
Even though each noun is singular by itself, together they form a plural subject:
- putovnica i karta = a passport and a ticket
So Croatian uses su.
Potrebne agrees with putovnica i karta.
Both nouns are:
- feminine
- singular individually
- but plural together because they are joined by i (and)
So the adjective must be feminine plural:
- potrebna = needed/necessary (feminine singular)
- potrebne = needed/necessary (feminine plural)
That is why the sentence has su potrebne.