Nećakinja još nosi pelenu, ali duda joj više ne treba preko dana.

Because pelenu is the accusative singular form of pelena.

In the sentence, nosi means wears / is wearing, and the thing being worn is the direct object, so it goes into the accusative:

• pelena = a diaper (dictionary form, nominative)
• nosi pelenu = wears a diaper

This is a very common pattern in Croatian:

• nosi kaput = he/she wears a coat
• nosi haljinu = he/she wears a dress
• nosi pelenu = he/she wears a diaper

Because pelena is a feminine noun ending in -a, its accusative singular is -u.

Yes. In Croatian, nositi is commonly used for things you wear on the body, and that includes a diaper.

So:

• nosi pelenu literally means wears a diaper
• in natural English, you might translate it as still wears a diaper or is still in diapers, depending on context

Croatian often uses the straightforward verb nositi where English sometimes prefers a more idiomatic phrase.

Here još means still.

So:

• Nećakinja još nosi pelenu = The niece still wears a diaper

Other common meanings of još include:

• more
• another
• yet

But in this sentence, still is the right meaning because it shows that the situation continues.

Više ne treba means no longer needs or doesn’t need anymore.

Breakdown:

• više = more / anymore
• ne treba = does not need / is not needed

Together:

• više ne treba = is no longer needed

In this sentence:

• duda joj više ne treba = she doesn’t need the pacifier anymore / the pacifier is no longer needed for her

This is a very common Croatian pattern:

• Više ne radim tamo. = I don’t work there anymore.
• Više ne puši. = He/She doesn’t smoke anymore.

Joj is the dative singular form of ona (she / her), and here it means to her / for her.

So:

• duda joj ne treba literally = the pacifier is not needed to/for her
• natural English = she doesn’t need the pacifier

This is a very common Croatian structure with trebati:

• Treba mi voda. = I need water.
  Literally: Water is needed to me.
• Treba ti pomoć. = You need help.
• Ne treba joj duda. = She doesn’t need a pacifier.

So joj refers back to nećakinja.

Because Croatian very often uses trebati in an impersonal-style structure:

• Treba mi... = I need...
• Treba ti... = You need...
• Treba joj... = She needs...

So instead of structuring it like English she needs X, Croatian often structures it more like X is needed to/for her.

That is why:

• duda joj više ne treba is very natural

You may also encounter other patterns with trebati, but for everyday needs, this dative pattern is extremely common and important.

Because in this construction, the thing needed is grammatically the subject.

So in:

• duda joj više ne treba

the subject is duda (the pacifier), and the verb agrees with it:

• duda = singular
• treba = singular

You can think of it literally as:

• The pacifier is no longer necessary for her.

That is why the verb is not agreeing with joj. Joj is an indirect object in the dative, not the subject.

Duda means pacifier. Depending on the variety of English, it may also be translated as:

• dummy (common in British English)
• soother
• binky (informal American English)

So:

• duda joj više ne treba preko dana = she doesn’t need the pacifier during the day anymore

Preko dana means during the day / in the daytime.

It is a very common expression:

• Radim preko dana. = I work during the day.
• Dijete spava preko dana. = The child sleeps during the day.

You could also say danju, which also means by day / during the day, but preko dana often sounds a bit more everyday and conversational in many contexts.

So here:

• više ne treba preko dana = doesn’t need it during the day anymore

This also implies that maybe she still needs it at night.

Yes, Croatian word order is flexible, but not random.

The given order sounds natural and neutral:

• duda joj više ne treba preko dana

A few things are happening:

1. joj is a clitic-like short pronoun form, and these short forms usually appear early in the sentence.
2. više ne treba is a very natural sequence for no longer needs / is no longer needed.
3. preko dana comes at the end as a time expression.

You may also hear variations such as:

• Duda joj preko dana više ne treba.
• Preko dana joj duda više ne treba.

These can shift emphasis slightly, but the meaning stays basically the same.

Ali is the most common everyday word for but in Croatian.

So:

• ..., ali ... = ..., but ...

Other words can sometimes express contrast too, such as nego or međutim, but they are used differently:

• ali = standard simple but
• međutim = however
• nego often appears after a negation in specific contrast structures

In this sentence, ali is exactly the normal choice:

• She still wears a diaper, but she doesn’t need a pacifier during the day anymore.

Because Croatian has no articles like English a / an / the.

So:

• nećakinja can mean a niece or the niece, depending on context
• pelenu can mean a diaper or the diaper
• duda can mean a pacifier or the pacifier

Context tells you which meaning is intended. In this sentence, English naturally uses the niece, a diaper, and the pacifier or just a pacifier, depending on how you translate it.

Nećakinja means niece, and it is the subject of the first clause:

• Nećakinja još nosi pelenu = The niece still wears a diaper

It is in the nominative singular, which is the basic subject form.

Then in the second clause, Croatian does not repeat nećakinja. Instead, it uses joj (to her / for her) to refer back to the same person:

• ali duda joj više ne treba preko dana

So the sentence avoids repetition in a very natural Croatian way.

Yes, and it is one of the nicest features of the sentence.

The two clauses are deliberately contrasted:

• još nosi pelenu = still wears a diaper
• više ne treba duda = no longer needs a pacifier

So the sentence compares two developmental facts:

• one thing is still true
• the other is no longer true

That contrast is reinforced by ali (but). This makes the sentence feel very natural and expressive.