Dječak pokazuje na dugu iznad parka.

Because dječak is the subject of the sentence, so it is in the nominative singular.

• Dictionary form: dječak = boy
• Nominative singular is the form used for who is doing the action

Here, the boy is doing the pointing, so dječak stays in its basic subject form.

In Croatian, the present tense can cover both ideas:

• he points
• he is pointing

So pokazuje can mean either one, depending on context.

English often requires a choice between simple present and present continuous, but Croatian usually does not. That is why one Croatian form can match two different English forms.

Pokazuje is the 3rd person singular present form of pokazivati.

So:

• ja pokazujem = I point / I am pointing
• ti pokazuješ = you point
• on/ona pokazuje = he/she points

In this sentence, dječak is he, so the verb is pokazuje.

A useful extra point: pokazivati is the imperfective verb, which fits an ongoing action like is pointing.

Because Croatian normally uses pokazivati na + accusative for pointing at something.

So:

• pokazivati na nešto = to point at something

This is just the standard Croatian pattern.
English uses point at, and Croatian uses pokazivati na.

Without na, pokazivati more often means to show:

• Pokazuje sliku. = He is showing a picture.
• Pokazuje na sliku. = He is pointing at the picture.

Because na here requires the accusative case, and duga changes to dugu in the accusative singular.

• nominative: duga = rainbow
• accusative: dugu

So:

• na dugu = at the rainbow

This is a very common Croatian pattern: the noun changes form because the preposition and the verb require a certain case.

Because iznad means above, and it takes the genitive case in Croatian.

• park = park
• genitive singular: parka

So:

• iznad parka = above the park

This is something you often have to memorize with prepositions in Croatian: each preposition tends to require a certain case.

In this sentence, it most naturally describes the rainbow:

• the rainbow above the park

So the structure is understood as:

• Dječak pokazuje na [dugu iznad parka].

In other words, the boy is pointing at a rainbow, and that rainbow is above the park.

Because Croatian does not have articles like English a and the.

So Croatian simply says:

• dječak = boy / the boy
• duga = rainbow / the rainbow
• park = park / the park

The exact meaning depends on context.
In this sentence, English naturally uses the boy, the rainbow, and the park, but Croatian does not need separate words for that.

Yes, Croatian word order is more flexible than English word order, because case endings show what each word is doing.

The neutral, ordinary order here is:

• Dječak pokazuje na dugu iznad parka.

That sounds natural and straightforward.

Other orders are possible, but they usually sound more marked, poetic, or contrastive. For example, moving na dugu to the front could emphasize what he is pointing at.

Still, learners should usually keep the neutral order until they are comfortable with Croatian emphasis patterns.

A rough English-friendly pronunciation is DYEH-chak.

Helpful parts:

• dj / dje sounds like a soft dy sound
• č sounds like ch in church
• a is a clear short a, not reduced
• the stress is usually on the first syllable: DJE-čak

Also useful:

• j in Croatian is pronounced like y in yes

So Dječak does not sound like English jack or jee-chak. It is closer to DYEH-chak.

Good question: duga can also be a form of the adjective dug = long.

But in this sentence, dugu is understood as the noun rainbow, because of the whole structure:

• pokazuje na dugu = points at the rainbow

So even though the form may remind you of the adjective long, here it is clearly the noun rainbow.