Breakdown of Oni idu na izlet.
oni
they
na
on, at / to, onto (with motion) / about (thinking about)
ići
to go
izlet
trip (m)
Questions & Answers about Oni idu na izlet.
Why do we use na instead of u for going on a trip?
In Croatian, you use na (instead of u) when your destination is an event, an activity, or an open space. Since izlet (a trip or excursion) is considered an activity rather than a physical enclosed building, it takes na.
The lesson says destinations use the accusative case, but izlet looks exactly like its dictionary form. Why didn't it change?
You are right that destinations after na need the accusative case! However, izlet is a masculine noun that is inanimate (a thing, not a living being). For masculine inanimate nouns, the accusative form is exactly the same as the dictionary form. So it actually is in the accusative case here, but the word itself doesn't get a new ending.
Can we drop the word Oni and just say Idu na izlet?
Yes, absolutely! Croatian often drops subject pronouns like Oni (they) because the verb itself already tells us who is doing the action. Since idu specifically means "they go" or "they are going", you only need to include Oni if you want to add extra emphasis to who is going.
The dictionary word for "to go" is ići. Why does it change to idu here?
The verb ići is very irregular in the present tense. For the "they" form, the correct word is idu. It does not follow the regular verb ending rules, so it is one of those important verb forms you simply have to memorize.