Porezala sam prst dok sam rezala kruh, ali rana nije duboka.

This is a very common Croatian pattern: the sentence uses aspect.

• rezati = to cut, to be cutting → imperfective
• porezati se / porezati = to cut oneself / to get cut → perfective

So:

• dok sam rezala kruh = while I was cutting bread
  This describes an ongoing action.
• porezala sam prst = I cut my finger / I got a cut on my finger
  This describes a completed event that happened during that action.

English often uses different tenses to show this contrast (was cutting vs cut), while Croatian often uses verb aspect.

Because there are really two past-tense verb phrases:

• Porezala sam prst
• dok sam rezala kruh

Croatian past tense is usually made with:

• the past participle +
• a form of biti (to be) as an auxiliary

Here:

• porezala sam = I cut
• sam rezala = I was cutting

So each clause needs its own auxiliary.

Because sam is a clitic in Croatian. Clitics usually go in the second position of the clause, not necessarily directly before the main verb as in English.

So Croatian prefers:

• Porezala sam prst.

rather than:

• Sam porezala prst. ❌

You can also say:

• Ja sam porezala prst.

if you want to emphasize I.

A useful thing to remember: Croatian clitics like sam, si, je, smo, ste, su often come after the first stressed word or phrase.

That ending shows the speaker is feminine singular in the past tense.

Croatian past participles agree with the subject in gender and number.

For a female speaker:

• porezala sam
• rezala sam

For a male speaker, it would be:

• porezao sam prst dok sam rezao kruh, ali rana nije duboka.

So this sentence tells you the speaker is a woman.

Because Croatian often drops subject pronouns when they are already clear from the verb form.

In:

• porezala sam
• sam rezala

the form sam plus the participle already tells you the subject is I.

So ja is unnecessary unless you want emphasis or contrast:

• Ja sam porezala prst, ne ti. = I cut my finger, not you.

This is normal in Croatian and many other Slavic languages.

Here dok means while.

• dok sam rezala kruh = while I was cutting bread

It introduces an action happening at the same time as another action.

By contrast, kad usually means when:

• Kad sam rezala kruh, porezala sam prst. = When I was cutting bread, I cut my finger.

In many situations dok and kad can both be possible, but dok especially emphasizes simultaneous ongoing action: while.

Because prst is the direct object, and for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: prst
• accusative: prst

That is why the form does not change in:

• Porezala sam prst.

The same thing happens with kruh in:

• rezala kruh

because kruh is also masculine inanimate, so its accusative singular is also the same as the nominative.

Because rana is the subject of that clause:

• rana nije duboka = the wound is not deep

Subjects normally go in the nominative case.

So:

• rana = nominative singular
• duboka = adjective agreeing with rana in gender, number, and case

Since rana is a feminine singular nominative noun, the adjective must also be feminine singular nominative:

• duboka

If rana were an object, then you might see a different case, but here it is the subject.

Because it agrees with rana, which is a feminine noun.

In Croatian, adjectives must match the noun in:

• gender
• number
• case

So:

• rana = feminine singular nominative
• duboka = feminine singular nominative

If the noun were masculine, you would get something like:

• rez nije dubok

If it were neuter:

• mjesto nije duboko

In Croatian, negation with biti (to be) is usually written as one word:

• nije = is not
• nisam = am not
• nisi = are not
• nismo = are not, etc.

So:

• rana nije duboka = the wound is not deep

This is just the normal Croatian negative form of je (is).

Yes, this is something English speakers often notice. In prst, the r acts like a syllable by itself. Croatian can have a syllabic r.

So prst is pronounced roughly like one syllable, something like:

• prst

not with a full vowel inserted in careful standard pronunciation.

English speakers often want to say something like perst or prist, but Croatian normally keeps it as a single syllable with syllabic r.

The same kind of thing happens in some other Croatian words too.

It is a standard Croatian word, but learners should know that some regional variation exists.

In standard Croatian:

• kruh = bread

In some neighboring varieties, especially Serbian, you will often see:

• hleb / хлеб

So for Croatian, kruh is the expected form.

Yes, depending on what you want to emphasize.

• Porezala sam prst. = I cut my finger.
• Porezala sam se. = I cut myself.

The version with prst is more specific, because it tells you exactly what part was injured.

You can even hear combinations like:

• Porezala sam se po prstu.

But in your sentence, Porezala sam prst is a very natural way to say I cut my finger.