U parku smo vidjeli vjevericu kako nosi mali kesten.

Because u parku expresses location: in the park.

In Croatian, u can be used with:

• the locative for being somewhere: u parku = in the park
• the accusative for motion into somewhere: u park = into the park

So:

• U parku smo vidjeli... = we saw ... in the park
• Išli smo u park. = we went to/into the park

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

Here, smo vidjeli already tells you the subject is we:

• smo = we are/have as an auxiliary in the past tense
• vidjeli = past participle

So (Mi) smo vidjeli means we saw, but mi is usually unnecessary unless you want emphasis:

• Mi smo vidjeli vjevericu... = We saw the squirrel (not someone else)

This is the standard Croatian past tense formation.

It is made with:

• the present tense of biti (to be) as an auxiliary
• the l-participle of the main verb

Here:

• smo = auxiliary for we
• vidjeli = past participle of vidjeti (to see)

So:

• ja sam vidio/vidjela
• ti si vidio/vidjela
• mi smo vidjeli/vidjele

Together, smo vidjeli = we saw

The participle in the Croatian past tense agrees with the gender and number of the subject.

Since the subject is we, the form depends on who we are:

• vidjeli = masculine plural, or a mixed-gender group
• vidjele = feminine plural

So this sentence assumes:

• either a group of men
• or a mixed group
• or uses the masculine plural as the default written form

If the speakers were all female, it would be:

• U parku smo vidjele vjevericu kako nosi mali kesten.

Because vjevericu is the accusative singular form, and here squirrel is the direct object of saw.

The dictionary form is:

• vjeverica = squirrel

But after vidjeti (to see), the thing seen is a direct object, so you use the accusative:

• vidjeli smo vjevericu = we saw a squirrel

This is a very common pattern:

• Imam knjigu. = I have a book.
• Vidim kuću. = I see a house.

Here kako nosi means something like:

• carrying
• as it was carrying
• how it was carrying

After verbs of perception like see, Croatian often uses kako + present tense to describe what someone or something was doing when seen.

So:

• vidjeli vjevericu kako nosi mali kesten = saw a squirrel carrying a small chestnut

This structure is very natural in Croatian.

A similar idea:

• Vidio sam ga kako trči. = I saw him running.
• Čuli smo je kako pjeva. = We heard her singing.

Because in Croatian, after verbs like saw, heard, or watched, the action being observed is often expressed with kako + present tense.

So although the main event is past (we saw), the present tense in nosi describes the action as it was happening at that moment.

English does something similar with:

• We saw a squirrel carrying a small chestnut.
• We saw a squirrel as it was carrying a small chestnut.

So the tense difference is normal and natural.

Because mali kesten is the direct object of nosi (carries), so it is in the accusative singular.

However, for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: mali kesten
• accusative: mali kesten

That is why the form does not change visibly here.

Compare:

• kesten je mali = the chestnut is small (nominative)
• nosi mali kesten = it carries a small chestnut (accusative, same form)

Yes. In Croatian, adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• kesten is masculine singular
• it is in the accusative
• it is inanimate

So the adjective must match:

• mali kesten

If the noun changed, the adjective would change too:

• mala vjeverica = a small squirrel
• malu vjevericu = a small squirrel (accusative)
• mali kesten = a small chestnut

Yes, Croatian word order is quite flexible.

Both are possible:

• U parku smo vidjeli vjevericu kako nosi mali kesten.
• Vidjeli smo vjevericu kako nosi mali kesten u parku.

Starting with U parku puts the location first, which gives it a bit more focus, like:

• In the park, we saw...

Also, smo is a clitic, so it tends to appear in the second position of the clause. That is why after the opening phrase U parku, you get:

• U parku smo vidjeli...

The base forms are:

• vidjeti = to see
• nositi = to carry

In the sentence, they appear as:

• vidjeli = past participle of vidjeti
• nosi = 3rd person singular present of nositi

So literally:

• smo vidjeli = we saw
• nosi = it carries / is carrying

Yes, vjeverica is a feminine noun.

A common clue is the ending -a, which often marks feminine nouns in Croatian.

You can see that in the case change:

• nominative: vjeverica
• accusative: vjevericu

That pattern is very typical of many feminine nouns:

• žena → ženu
• mačka → mačku
• vjeverica → vjevericu

Yes, but kako is very natural here.

For example, Croatian can sometimes use a relative clause or another structure depending on style and meaning, but with perception verbs, kako + present tense is one of the most common and idiomatic ways to say saw someone doing something.

So for a learner, this is a very useful pattern to remember:

• vidjeti nekoga kako + present
• čuti nekoga kako + present

Examples:

• Vidio sam dijete kako plače. = I saw a child crying.
• Čuli smo psa kako laje. = We heard a dog barking.