Djeca su vidjela patku, ali je nisam stigla fotografirati dok je ulazila u vodu.

Because it is the direct object of vidjela. In Croatian, a feminine noun ending in -a usually changes to -u in the accusative singular:

• patka = nominative
• patku = accusative

So Djeca su vidjela patku means The children saw a duck.

Here je is an unstressed object pronoun meaning it or her, referring back to patku.

Even though English would normally say it for a duck, Croatian uses the pronoun that matches the noun’s grammatical gender. Since patka is feminine, the pronoun is feminine too.

So:

• patku = a duck
• je = it / her referring to that duck

Yes. The feminine singular accusative clitic can be je or ju.

So you may hear or see both:

• ali je nisam stigla fotografirati
• ali ju nisam stigla fotografirati

Both mean the same thing here. In this sentence, je is completely natural and standard.

Because je is a clitic, and Croatian clitics usually stand very near the beginning of the clause, often in the second position.

In this clause, the first word is ali, so the clitic je comes right after it:

• ali je nisam stigla fotografirati

This word order is very typical in Croatian.

They are doing two different jobs.

1. ali je nisam stigla fotografirati
   Here je is a pronoun meaning it/her.

2. dok je ulazila u vodu
   Here je is the auxiliary verb to be, used to form the past tense.

So the same word form appears twice, but with different meanings and functions.

Because djeca is the irregular plural of dijete and it does not use the masculine plural participle vidjeli.

With djeca, Croatian uses:

• plural auxiliary: su
• participle form: vidjela

So:

• Djeca su vidjela = correct
• Djeca su vidjeli = not correct here

This is a pattern worth memorizing: djeca su došla, djeca su vidjela, etc.

Yes. Stigla shows that the speaker is feminine singular.

Croatian past tense participles agree with the subject, and here the subject is an omitted I:

• nisam stigla = I didn’t manage said by a woman
• nisam stigao = the same thing said by a man

So the sentence tells you something about the speaker, even though ja is not written.

Because Croatian often leaves out subject pronouns when the verb already makes the person clear.

• nisam already shows I did not
• stigla adds feminine singular agreement

So ja is understood and does not need to be said unless the speaker wants emphasis.

The verb stići here does not mean to arrive. In this pattern, stići + infinitive means:

• to manage to do something
• to get the chance to do something
• to have enough time to do something

So nisam stigla fotografirati means I didn’t manage to photograph or I didn’t get a chance to photograph.

Because after stići in this meaning, Croatian uses an infinitive directly:

• stići vidjeti
• stići napraviti
• stići fotografirati

English usually uses to + verb after manage, but Croatian just uses the bare infinitive form.

Because dok here means while, and the action is presented as ongoing.

• ulazila is imperfective and describes a process in progress: while it was going into / entering
• ušla would be perfective and would sound more like a completed event: when it went in / entered

So dok je ulazila u vodu gives the idea that the duck was in the middle of entering the water when the speaker failed to get the photo.

Also, ulazila is feminine singular because the subject of that clause is the duck, patka.

Because Croatian uses different cases with u depending on whether there is movement or location.

• u + accusative = movement into
• u + locative = being in

So:

• ulazila u vodu = was going into the water
• bila u vodi = was in the water

Here the duck is moving into the water, so vodu is correct.