Na ključevima imam mali crveni privjesak.

Croatian word order is flexible. Starting with Na ključevima puts the focus on the location: on the keys.

A more neutral order would be:

Imam mali crveni privjesak na ključevima.

Both are correct. The original sentence feels a bit like: On my keys, I have a small red charm.

Ključevima is in the locative plural here.

The preposition na takes:

• locative when it means location, as in on / on top of / attached to
• accusative when it means movement onto something

In this sentence, the charm is already located on the keys, so Croatian uses the locative:

• na ključevima = on the keys

The ending -ima is a very common plural ending for dative, locative, and instrumental, so the preposition tells you which function it has here.

Because the sentence is talking about a set of keys, not one single key.

• na ključu = on the key
• na ključevima = on the keys

In everyday life, a keychain charm is usually attached to a bunch of keys, so the plural sounds natural.

Because the verb already shows the subject.

Imam means I have, so Croatian does not need ja unless you want emphasis or contrast.

• Imam mali crveni privjesak. = I have a small red charm.
• Ja imam mali crveni privjesak. = I have a small red charm, with extra emphasis on I

Dropping subject pronouns is very normal in Croatian.

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

So:

• nominative: privjesak
• accusative: privjesak

Since imam takes a direct object, privjesak is accusative here, even though it looks identical to the dictionary form.

Because adjectives in Croatian must agree with the noun in gender, number, and case.

Here, privjesak is:

• masculine
• singular
• accusative

So the adjectives match it:

• mali
• crveni
• privjesak

Since privjesak is masculine inanimate, its accusative looks like the nominative, and the adjectives also appear in the matching masculine singular form.

Mali crveni privjesak is the most natural order.

In Croatian, adjective order is somewhat flexible, but speakers usually prefer a more natural descriptive sequence, and size before color is very common:

• mali crveni privjesak = small red charm

You may sometimes hear a different order for stylistic emphasis, but mali crveni is the ordinary choice.

Yes. In Croatian, na can be used for something physically attached to or hanging from something else, not only something resting on top.

So na ključevima is a natural way to say that the charm is on your keys, meaning attached to the keyring or key set.

If you said uz ključeve, that would mean more like with the keys in a general sense, not necessarily attached.

Croatian often leaves possession unstated when it is obvious from context.

Because the sentence says imam = I have, it is very natural to understand the keys as the speaker’s keys.

If you want to make it explicit, you could say:

• Na svojim ključevima imam mali crveni privjesak.

Here svojim is especially natural because it refers back to the subject of the sentence. Using mojim is also possible, but svojim is often preferred in this kind of sentence.

Yes, absolutely. That is a very natural sentence too.

The difference is mainly one of emphasis:

• Na ključevima imam mali crveni privjesak.
  Focus first on the keys / location.

• Imam mali crveni privjesak na ključevima.
  More straightforward, neutral statement.

Both are correct and normal.

A rough guide:

• ključevima ≈ KLYOO-che-vee-ma
• privjesak ≈ PREEV-ye-sak

A few useful sounds:

• č sounds like ch in church
• j sounds like y in yes
• lj is a soft sound somewhat like the lli in some pronunciations of million, though not exactly the same

So:

• klju is not like English clue, but a softer klyu
• vje in privjesak sounds like vye