Moja sestra se porezala dok je rezala mrkvu za juhu.

Because sestra is the subject of the sentence, so it is in the nominative case.

• sestra = sister
• moja = my

The possessive moja has to agree with sestra in:

• gender: feminine
• number: singular
• case: nominative

So moja sestra means my sister.

If it were in a different case, both words could change, for example:

• Vidim moju sestru = I see my sister

Se is a reflexive clitic.

In this sentence, porezala se means cut herself or got cut.

So:

• rezati = to cut
• porezati se = to cut oneself / to get cut

It does not always translate word-for-word as oneself, but here that is the idea. Without se, the meaning would be different.

This is mainly about aspect, which is very important in Croatian.

• rezati is imperfective: it describes an ongoing, repeated, or uncompleted action
• porezati se is perfective: it describes a completed event, here a single accident

So the sentence means:

• she was cutting carrots = ongoing action
• she cut herself = one completed event that happened during that action

That is why Croatian uses:

• dok je rezala = while she was cutting
• se porezala = she cut herself

Because the subject is feminine singular: sestra.

In the Croatian past tense, the main verb form agrees with the subject in gender and number.

So with a feminine singular subject, you get:

• porezala
• rezala

Compare:

• masculine singular: porezao, rezao
• feminine singular: porezala, rezala
• neuter singular: porezalo, rezalo
• plural forms also change

A male speaker talking about himself would say porezao sam se, while a female speaker would say porezala sam se.

This is a very common learner question.

In the past tense, Croatian often uses an auxiliary such as sam, si, je, smo, ste, su. But with reflexive verbs in the 3rd person singular, Croatian normally says:

• se porezala

not:

• se je porezala

So Moja sestra se porezala is the normal form.

But in dok je rezala, the verb is not reflexive, so the auxiliary je appears normally.

So the contrast is:

• se porezala = normal reflexive 3rd person singular past form
• je rezala = normal non-reflexive 3rd person singular past form

Because mrkvu is the accusative singular, used here for the direct object.

• nominative: mrkva = carrot
• accusative: mrkvu = carrot

She was cutting what?
mrkvu

For many feminine nouns ending in -a, the accusative singular ends in -u:

• juha → juhu
• mrkva → mrkvu
• sestra → sestru

Because the preposition za here takes the accusative case and means for.

• juha = soup
• juhu = soup in the accusative

So:

• za juhu = for soup

This phrase means she was cutting the carrot for soup, that is, for use in making soup.

Not necessarily.

In Croatian, singular nouns are often used in a general or ingredient-like way, especially in cooking contexts.

So rezala mrkvu za juhu can naturally mean:

• cutting carrot for soup
• cutting a carrot for soup

It does not strongly force the idea of exactly one whole carrot in the way English sometimes does.

Dok means while here.

It introduces a time clause:

• dok je rezala mrkvu za juhu = while she was cutting carrot for soup

It sets the background action during which the main event happened.

So the structure is:

• main event: Moja sestra se porezala
• background action: dok je rezala mrkvu za juhu

Croatian word order is fairly flexible, but short words like se and je are clitics. Clitics usually cannot stand in the first position of a clause and tend to appear near the beginning.

So:

• Moja sestra se porezala = natural
• Se moja sestra porezala = not correct

You may also hear or see slightly different orders depending on style and emphasis, but se cannot just go anywhere.

Yes. Croatian allows more flexibility than English.

For example, you can also say:

• Dok je rezala mrkvu za juhu, moja sestra se porezala.

The meaning stays basically the same, but the focus changes a little:

• starting with dok... emphasizes the background situation first
• starting with Moja sestra... emphasizes the main event first

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

So:

• sestra can mean a sister or the sister, depending on context
• mrkvu can mean a carrot or the carrot
• juhu can mean soup or the soup

In this sentence, moja sestra already makes the reference definite enough, because my sister is specific.

Croatian does not have a direct equivalent of the English past continuous as a separate tense form.

Instead, Croatian usually expresses this idea through:

• aspect
• context
• sometimes words like dok

So dok je rezala is understood as while she was cutting, even though the form is literally just past tense with an imperfective verb.

That is why the sentence works so naturally:

• rezala gives the ongoing/background action
• porezala se gives the completed event that happened during it