Dokumentarni film i odjavna špica dugo traju.

Because the subject is made of two things joined by i (and):

• dokumentarni film = documentary film
• odjavna špica = end credits / closing titles

In Croatian, when two singular nouns are connected by i, the verb normally goes into the plural:

• film traje = the film lasts
• špica traje = the credits last
• film i špica traju = the film and the credits last

So traju is the 3rd person plural form of trajati (to last).

Because dugo here is an adverb, not an adjective.

It modifies the verb traju (last), so it means for a long time / long:

• dugo traju = they last a long time

Compare:

• dug film = a long film
  
  • here dug is an adjective describing a noun
• film dugo traje = the film lasts a long time
  
  • here dugo is an adverb describing the action

This is very common in Croatian:

• brz = fast (adjective)
• brzo = quickly (adverb)

They are in the nominative case, because they are the subject of the sentence.

You can tell because they are the things doing the action of traju:

• dokumentarni film = nominative singular
• odjavna špica = nominative singular

The sentence structure is basically:

• [subject] + [subject] + [verb]

So both nouns stay in nominative.

Because adjectives in Croatian must agree with the noun they describe in gender, number, and case.

Here:

• film is masculine singular
• špica is feminine singular

So the adjectives match them:

• dokumentarni film
  
  • dokumentarni = masculine singular nominative
• odjavna špica
  
  • odjavna = feminine singular nominative

This is normal adjective agreement in Croatian.

It usually means the end credits or closing titles of a film or TV program.

Related expression:

• uvodna špica = opening credits / intro titles
• odjavna špica = closing credits / end credits

So in this sentence, odjavna špica refers to the part at the end where the credits roll.

Yes, you could absolutely say:

• Dokumentarni film i odjavna špica traju dugo.

That is often the more neutral word order for English speakers, because the adverb comes after the verb.

But Croatian word order is relatively flexible. Both versions are natural:

• Dokumentarni film i odjavna špica dugo traju.
• Dokumentarni film i odjavna špica traju dugo.

The difference is mostly one of style or focus, not basic meaning.

Because Croatian does not have articles like the or a/an.

So:

• dokumentarni film can mean a documentary film or the documentary film
• odjavna špica can mean end credits or the end credits

The exact meaning depends on context.

This is one of the big differences from English.

The base form is trajati (to last).

In the present tense:

• ja trajem = I last
• ti traješ = you last
• on / ona / ono traje = he / she / it lasts
• mi trajemo = we last
• vi trajete = you (plural/formal) last
• oni / one / ona traju = they last

So in your sentence, traju agrees with the plural subject dokumentarni film i odjavna špica.

Yes. In natural English, you would usually translate dokumentarni film simply as documentary.

Word for word, it is:

• dokumentarni = documentary
• film = film

But in real usage, Croatian often keeps both words where English might shorten it:

• gledam dokumentarni film = I’m watching a documentary

So learners should know both the literal structure and the natural English equivalent.

Here i means and, but it also matters grammatically because it joins two nouns into a compound subject.

That is why the verb becomes plural:

• film traje = the film lasts
• špica traje = the credits last
• film i špica traju = the film and the credits last

So i does not change the meaning in a surprising way, but it does change verb agreement.