Na izletu smo opet vidjeli lisicu, a navečer i sovu iznad ceste.

Because na izletu is in the locative case and means on the trip / during the outing.

With na, Croatian often uses:

• accusative for movement toward something: Idemo na izlet = We are going on a trip
• locative for being at or during something: Na izletu smo... = On the trip / While on the trip, we...

So here the sentence is describing what happened during the outing, not motion toward it.

Croatian often leaves out subject pronouns when they are already clear from the verb.

Here, smo tells you the subject is we, because smo is the 1st person plural auxiliary form of biti used in the past tense.

So:

• smo vidjeli = we saw
• mi smo vidjeli = we saw too, but with extra emphasis on we

The pronoun mi is unnecessary unless you want contrast or emphasis.

This is because smo is a clitic, and Croatian clitics usually go in second position in the clause.

So in Na izletu smo opet vidjeli lisicu, the first unit is Na izletu, and then the clitic smo comes right after it.

That is very normal Croatian word order.

You may also hear other arrangements in real speech, but Na izletu smo opet vidjeli... is very natural and standard.

Vidjeli is the past active participle used to form the Croatian past tense together with smo.

So:

• smo vidjeli = we saw

The form vidjeli is plural masculine/mixed-gender. Croatian past participles agree with the subject in number and sometimes gender.

Examples:

• Mi smo vidjeli = We saw (male group or mixed group, or gender not specified)
• Mi smo vidjele = We saw (all-female group)

So the sentence either refers to a mixed group, a male group, or just uses the default plural form.

Because they are direct objects, so they are in the accusative case.

The verb vidjeti takes a direct object in the accusative:

• vidjeti lisicu = to see a fox
• vidjeti sovu = to see an owl

Both lisica and sova are feminine nouns, and in the singular accusative they change from -a to -u:

• lisica → lisicu
• sova → sovu

Here a is a linking word that can mean something like and, while, or but, depending on context.

In this sentence, it connects two parts of the story:

• first, they saw a fox again
• later in the evening, they also saw an owl

So here a is best understood as a natural connector such as and then / and rather than a strong but.

Croatian a often marks a shift or new piece of information, even when there is no real contrast.

Here i means also / too, not just plain and.

The idea is:

• we saw a fox
• and in the evening, also an owl

So i sovu means an owl too or also an owl.

Its position matters: i often stands right before the word it emphasizes. Here it highlights sovu.

Yes, that would be the full version:

• Na izletu smo opet vidjeli lisicu, a navečer smo vidjeli i sovu iznad ceste.

But Croatian, like English, often omits repeated words when they are easy to understand from context. This is called ellipsis.

So the shorter version sounds natural and avoids repetition:

• ..., a navečer i sovu iznad ceste.

The omitted part is understood as smo vidjeli.

Navečer means in the evening or toward evening, depending on context.

In this sentence it means in the evening.

It functions as an adverb, so it does not change form here. It is a very common time expression.

Compare:

• ujutro = in the morning
• popodne = in the afternoon
• navečer = in the evening

Iznad requires the genitive case, so cesta changes to ceste.

• cesta = road
• iznad ceste = above the road

So:

• iznad
  • genitive

This is a very common preposition pattern in Croatian.

Other examples:

• iznad kuće = above the house
• iznad grada = above the city

Yes. Vidjeti is the normal verb for to see / to catch sight of, and it is typically treated as perfective in this kind of use.

That fits this sentence well, because it refers to specific completed sightings:

• they saw a fox
• later they saw an owl

Croatian often uses vidjeti when the event is a single, completed act of seeing.

A different verb like viđati would suggest repeated or habitual seeing, and gledati means to look at / watch, which is different from simply seeing something.

So vidjeli is exactly the natural choice here.