Na prstu imam mali flaster.

Because na is being used here to show a location: on the finger.

After na with a static location, Croatian normally uses the locative case:

• na prstu = on the finger

The base noun is prst = finger, but in the locative singular it becomes prstu.

A useful contrast:

• Na prstu imam mali flaster. = I have a small bandage on my finger.
• Stavio sam flaster na prst. = I put the bandage onto my finger.

In the second example, there is movement onto the finger, so na takes the accusative instead.

Croatian often leaves out possessive words like my, your, his, etc. when the meaning is obvious from context, especially with:

• body parts
• clothing
• things closely connected to the person

So:

• Na prstu imam mali flaster.
  naturally means
• I have a small bandage on my finger.

Croatian does not need to say my finger unless you want to emphasize it:

• Na mom prstu imam mali flaster. = I have a small bandage on my finger (more explicit/emphatic)

In everyday speech, the shorter version is more natural.

It is in the accusative singular, because it is the direct object of imam (I have).

• imam
  • object in the accusative

So:

• imam flaster = I have a bandage

Why does it still look like flaster and not change? Because flaster is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: mali flaster
• accusative: mali flaster

Same form, different grammatical role.

Because mali has to agree with flaster in:

• gender
• number
• case

The noun flaster is:

• masculine
• singular
• here in the accusative

So the adjective must match it:

• mali flaster

Compare:

• mali flaster = masculine
• mala rana = feminine
• malo mjesto = neuter

This agreement is a very important part of Croatian grammar.

Because na means on in the sense of being on a surface.

A bandage is physically on the finger, so Croatian uses:

• na prstu = on the finger

This is very natural with body parts too:

• na ruci = on the hand/arm
• na nozi = on the leg/foot
• na licu = on the face

If you used u, it would usually suggest in rather than on, which would not fit this meaning.

Because Croatian has no articles.

English uses:

• a
• the

Croatian does not. So mali flaster can mean:

• a small bandage
• the small bandage

The exact meaning depends on context.

If Croatian wants to be more specific, it can use other words such as demonstratives:

• taj mali flaster = that small bandage
• ovaj mali flaster = this small bandage

But there is no direct equivalent of English a/an/the.

No, Croatian word order is fairly flexible.

This sentence starts with Na prstu to highlight the location:

• Na prstu imam mali flaster.
  = On my finger, I have a small bandage.

A very natural alternative is:

• Imam mali flaster na prstu.

Both are correct. The difference is mainly about focus:

• Na prstu... puts emphasis on where
• Imam mali flaster... starts more neutrally with I have

Because Croatian uses case endings, word order can move around more than in English.

Yes. Flaster is a normal, common everyday word in Croatian.

It usually means an adhesive bandage / sticking plaster / Band-Aid-type plaster.

That is different from words like:

• zavoj = a bandage/wrap
• gaza = gauze

So flaster is specifically the kind that sticks to the skin.

Yes, you could.

• Imam mali flaster na prstu = I have a small bandage on my finger
• Nosim mali flaster na prstu = I’m wearing a small bandage on my finger

Both can be natural, but the nuance is slightly different:

• imam is a very common, simple way to describe the situation
• nosim emphasizes that you are wearing it

In many everyday situations, imam sounds completely normal and idiomatic.

Yes. A common alternative is:

• Na prstu mi je mali flaster.

Literally, this is closer to:

• There is a small bandage on my finger.

Here:

• mi = to me / my
• je = is

This structure focuses more on what is located there, while imam focuses more on I have.

So both are natural:

• Na prstu imam mali flaster.
• Na prstu mi je mali flaster.

They are very close in meaning, but the grammar is different.