Taj dječak sjedi ispod drveta, dok djevojčica gleda prema brdu.

Taj means that.

It is in the masculine singular nominative form because it describes dječak, which is a masculine singular noun and the subject of the sentence.

A useful comparison:

• ovaj dječak = this boy
• taj dječak = that boy
• onaj dječak = that boy over there

So taj has to match dječak in gender, number, and case.

Croatian does not have articles like English a or the.

So:

• djevojčica can mean a girl or the girl
• the exact meaning depends on context

If you want to be more specific, Croatian often uses:

• word order
• context
• demonstratives such as ova, ta, ona

So djevojčica by itself is completely normal.

Because ispod normally takes the genitive case.

The basic form of the noun is:

• drvo = tree or wood depending on context

But after ispod:

• drvo becomes drveta

So:

• ispod drveta = under the tree

This is a very important pattern:

• iznad kuće = above the house
• pored ceste = beside the road
• ispod stola = under the table

These prepositions commonly require the genitive.

Because prema takes the dative case.

The basic form is:

• brdo = hill

In the dative singular:

• brdo becomes brdu

So:

• prema brdu = toward the hill / in the direction of the hill

This is the normal grammar after prema.

There is a difference in meaning.

• gledati brdo would mean to look at the hill as a direct object
• gledati prema brdu means to look toward the hill or in the direction of the hill

So prema adds the idea of direction, not necessarily focused observation of the hill itself.

English has the same distinction:

• look at the hill
• look toward the hill

Here dok means while.

It connects two actions happening at the same time:

• the boy is sitting
• the girl is looking toward the hill

So it works like:

• while
• sometimes whereas, depending on context

Important: dok can also mean until in other sentences, but here it clearly means while, because it links two simultaneous present-tense actions.

They are both 3rd person singular present tense forms.

That is because the subjects are singular:

• taj dječak = one boy
• djevojčica = one girl

The dictionary forms are:

• sjediti = to sit
• gledati = to look/watch

Present tense here:

• on sjedi = he sits / he is sitting
• ona gleda = she looks / she is looking

Croatian present tense often covers both the simple present and the present continuous ideas that English separates.

Here is the breakdown:

• taj dječak → nominative
  It is the subject of sjedi

• drveta → genitive
  It comes after ispod

• djevojčica → nominative
  It is the subject of gleda

• brdu → dative
  It comes after prema

So the sentence is a good example of how Croatian cases are triggered by both:

• sentence role, and
• prepositions

Yes. Croatian word order is more flexible than English word order because cases show grammatical relationships.

The version you have is neutral and natural:

• Taj dječak sjedi ispod drveta, dok djevojčica gleda prema brdu.

But other orders are possible for emphasis, for example:

• Ispod drveta sjedi taj dječak...
• Djevojčica prema brdu gleda...
  This is possible, though less neutral.

Even though word order can move around, not every version sounds equally natural. The original sentence is a very standard, neutral choice.

A rough English-friendly guide:

• dječak ≈ dyeh-chak
• djevojčica ≈ dye-VOY-chee-tsa
• drveta ≈ DER-vե-ta or more tightly dr-VE-ta
• brdu ≈ BER-doo or tightly br-doo

A few key pronunciation points:

• j is pronounced like English y
• č sounds like ch in church
• c sounds like ts
• Croatian r can sometimes act almost like a vowel, which is why words like brdu may feel unusual to English speakers

The spelling is very regular in Croatian, so once you learn the sound of each letter, pronunciation becomes much easier.