Na koljenu imam malu ranu, pa mi je doktorica stavila flaster.

Because na can take different cases depending on the meaning:

• na + locative = on / at a place, with no movement
• na + accusative = onto / to a place, with movement

Here, the wound is located on the knee, so Croatian uses the locative:

• na koljenu = on the knee

Compare:

• Rana je na koljenu. = The wound is on the knee.
• Stavila je flaster na koljeno. = She put the bandage onto the knee.

Because koljeno is a neuter noun, and after na in this sentence it is in the locative singular.

The basic form is:

• koljeno = knee

Its locative singular form is:

• koljenu

So:

• na koljenu = on the knee

This is a normal pattern for many neuter nouns in -o or -e.

Because malu ranu is the direct object of imam.

The verb imati takes an object in the accusative case. Since rana is a feminine noun, both the noun and the adjective change form:

• mala rana = nominative, the basic dictionary form
• malu ranu = accusative

So:

• Imam malu ranu. = I have a small wound.

The adjective must agree with the noun in:

• gender
• number
• case

Ranu is accusative singular of rana.

Forms:

• rana = nominative singular
• ranu = accusative singular

In this sentence, it is accusative because it is the thing being had:

• imam ranu = I have a wound

This is a very common pattern after imam / imaš / ima.

Here pa means something like:

• so
• and so
• so then

It links the two parts of the sentence in a natural, conversational way:

• I have a small wound on my knee, so the doctor put a bandage on it.

In Croatian, pa is extremely common in speech and writing. It often sounds softer and more natural than a direct English-style therefore.

Mi here means to me or for me.

It is the unstressed dative form of ja:

• ja = I
• meni / mi = to me

So:

• doktorica mi je stavila flaster = the doctor put a bandage on me / for me

In English, we often do not say to me in this situation, but Croatian commonly uses the dative for the affected person.

Croatian often uses the dative to show the person affected by an action, especially with body parts, clothing, and personal items.

So instead of explicitly repeating my knee, Croatian can say:

• Na koljenu imam malu ranu, pa mi je doktorica stavila flaster.

Literally, this is something like:

• On the knee I have a small wound, so the doctor put me a bandage.

A more English-like version with an explicit possessive is also possible, but Croatian often prefers this more natural dative construction.

Because Croatian has clitics, which are short unstressed words like:

• mi
• je
• se
• ga
• sam

These usually go near the second position in the clause.

So in:

• pa mi je doktorica stavila flaster

the clitics mi je come early in the clause.

You may also hear:

• Pa doktorica mi je stavila flaster.

but the version in your sentence is very natural because it follows the usual clitic placement pattern.

This is the Croatian perfect tense, which is used for many past actions.

It is made with:

• a form of biti (je here)
• the past participle (stavila)

So:

• je stavila = put / has put

In everyday Croatian, this is the normal way to talk about a completed action in the past.

Because the participle agrees with the subject, and the subject is doktorica, which is feminine singular.

So:

• doktorica je stavila
• doktor je stavio
• dijete je stavilo

Croatian past participles agree in:

• gender
• number

That is why the ending matters.

Because doktorica is the feminine noun meaning female doctor.

Compare:

• doktor = male doctor
• doktorica = female doctor

Since the sentence is talking about a woman, doktorica is used, and that is also why the verb form is stavila.

It actually is in the accusative, but for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• flaster = nominative singular
• flaster = accusative singular

That is why there is no visible change here.

Compare with a masculine animate noun, where the accusative usually does change:

• doktor = nominative
• vidim doktora = accusative

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

So nouns like:

• rana
• doktorica
• flaster

can mean:

• a wound / the wound
• a doctor / the doctor
• a bandage / the bandage

The exact meaning depends on context.

In this sentence, English naturally uses a small wound and the doctor, but Croatian does not need separate words for that.

Yes, absolutely.

• Na koljenu imam malu ranu
• Imam malu ranu na koljenu

Both are correct.

The difference is mostly about focus:

• Na koljenu imam malu ranu puts a little more emphasis on where
• Imam malu ranu na koljenu starts more neutrally with I have

Both are natural Croatian.

Because with body parts, Croatian often leaves the possessive unstated when it is already obvious from context.

So:

• Na koljenu imam malu ranu naturally means I have a small wound on my knee

If needed, Croatian can make it explicit:

• Na mom koljenu imam malu ranu
• Na svojem koljenu imam malu ranu

But in ordinary speech, that is often unnecessary.