Na ruci nosi crnu narukvicu.

Because na ruci uses the locative case.

• Base form: ruka = hand/arm
• After na when it means on in a location sense, Croatian often uses the locative
• ruka → ruci in the locative singular

So:

• na ruci = on the hand / on the arm / on the wrist area

If you used ruka, that would just be the dictionary form, not the correct form after na here.

Because narukvicu is in the accusative singular, and the adjective has to match it.

• narukvica = bracelet
• Accusative singular: narukvicu
• crna = black
• To agree with narukvicu, it becomes crnu

So:

• crna narukvica = a black bracelet
• crnu narukvicu = a black bracelet as the direct object

The verb nosi takes a direct object, so the bracelet has to be in the accusative.

Because they do different jobs in the sentence.

• crnu narukvicu is the thing being worn, so it is the direct object of nosi → accusative
• na ruci tells you where the bracelet is being worn → locative after na

So the sentence has:

• nosi crnu narukvicu = wears a black bracelet
• na ruci = on the arm/hand/wrist

Different meanings, different cases.

Here nosi means wears.

The verb nositi has a broad meaning: to carry, to wear, to bear. The exact meaning depends on context.

In this sentence, because the object is narukvicu (bracelet), nosi naturally means is wearing or wears.

So:

• nosi torbu = carries a bag
• nosi kaput = wears a coat
• nosi crnu narukvicu = wears a black bracelet

Because Croatian often leaves out the subject pronoun when it is clear from the verb form.

Nosi can mean:

• he wears
• she wears
• sometimes it wears depending on context, though that is less common in real use

Croatian verbs already contain person and number information, so pronouns like on (he) and ona (she) are often omitted unless you want emphasis or contrast.

So this sentence could mean:

• He is wearing a black bracelet on his arm
• She is wearing a black bracelet on her arm

The sentence itself does not tell you which one.

It can mean both, depending on context.

Croatian ruka covers a wider area than English hand. In many situations it can refer to the hand or the arm in a general sense.

In na ruci nosi crnu narukvicu, English would often translate this more naturally as:

• on her wrist
• on his wrist
• on her arm

Since a bracelet is usually worn around the wrist, that is probably the intended meaning, even though Croatian says ruci.

Croatian word order is fairly flexible. This version is natural because it puts the location first for emphasis or scene-setting.

Possible versions include:

• Na ruci nosi crnu narukvicu.
• Nosi crnu narukvicu na ruci.
• Crnu narukvicu nosi na ruci.

They all mean roughly the same thing, but the focus changes slightly.

• Na ruci... emphasizes where
• Crnu narukvicu... emphasizes what
• Nosi... is the more neutral verb-first option in some contexts

So the given sentence is correct and natural, especially if the speaker wants to highlight the location first.

Yes, but it would mean something different.

• na ruci = on the arm/hand as a location, so locative
• na ruku = onto the arm/hand or toward it, so accusative

Compare:

• Narukvica je na ruci. = The bracelet is on the arm/wrist.
• Stavila je narukvicu na ruku. = She put the bracelet onto her arm/wrist.

So in your sentence, na ruci is correct because it describes where the bracelet is being worn, not movement onto the arm.

Because Croatian has no articles.

English distinguishes between:

• a black bracelet
• the black bracelet

Croatian usually does not mark that difference with a separate word. Context tells you whether something is definite or indefinite.

So crnu narukvicu could mean:

• a black bracelet
• the black bracelet

depending on the situation.

Narukvica specifically means bracelet.

It comes from ruka, which helps make the meaning easy to remember: it is something worn on the arm/hand area.

Related words:

• prsten = ring
• ogrlica = necklace
• naušnica = earring
• sat = watch

So crna narukvica is unambiguously a black bracelet.

It can be either, depending on context.

The present tense nosi can mean:

• what someone is wearing right now
• what someone usually wears

For example:

• Looking at a photo: Na ruci nosi crnu narukvicu. = He/She is wearing a black bracelet on the wrist.
• Describing a person’s usual style: Na ruci nosi crnu narukvicu. = He/She wears a black bracelet on the wrist.

Croatian present tense often covers both meanings, just like English can in some contexts.

Yes, and it is understandable, but it is slightly different in feel.

• nosi crnu narukvicu = is wearing a black bracelet
• ima crnu narukvicu na ruci = has a black bracelet on her/his arm

Nosi is the more natural verb when talking about clothes or accessories being worn. Ima is possible, but it sounds more like describing what is there rather than the act/state of wearing it.

So for clothing and accessories, nosi is usually the better choice.