Djeca lijepe naljepnice na fascikl i crtaju sunce žutim flomasterom.

Here it is a verb, not an adjective.

• lijep = beautiful
• lijepe can be a form of that adjective in some contexts
• but in this sentence, Djeca lijepe naljepnice..., lijepe means they stick / they glue

You can tell it is a verb because it follows the subject djeca and is followed by a direct object, naljepnice.

Djeca means children. It refers to more than one child, so in meaning it is plural.

It is the irregular plural of dijete (child). Because the subject is plural in meaning, the verbs are also plural:

• djeca lijepe
• djeca crtaju

So for a learner, the safest way to think of djeca is: it means children, so use a plural verb with it.

Lijepe is the 3rd person plural present tense of lijepiti (to stick, glue, paste).

So:

• ja lijepim = I stick/glue
• ti lijepiš = you stick/glue
• oni/one/ona lijepe = they stick/glue

In this sentence, djeca lijepe = the children are sticking / stick.

Because naljepnice is the direct object of lijepe: it is the thing being stuck.

The basic noun is:

• naljepnica = sticker

Here we need the plural because there is more than one sticker:

• naljepnice = stickers

In this sentence, the form is accusative plural, but for this noun that looks the same as the nominative plural. So even though the case has changed grammatically, the word does not visibly change.

Because na is used for putting something onto a surface.

Here the idea is:

• the stickers are being put onto the folder

So na fascikl means onto/on the folder.

This is different from:

• u fascikl = in/into the folder

If the stickers were inside the folder, you would not use na.

It actually is in the accusative, but you do not see a change because fascikl is a masculine inanimate noun.

For many masculine inanimate nouns in Croatian:

• nominative singular = accusative singular

So:

• fascikl = nominative
• fascikl = accusative

That is why na fascikl looks unchanged, even though the case is doing grammatical work.

Because Croatian, like English, often omits a repeated subject when it is the same.

So:

• Djeca lijepe naljepnice na fascikl i crtaju sunce...

means:

• The children stick stickers on the folder and draw the sun...

The subject djeca applies to both verbs:

• lijepe
• crtaju

You could repeat the subject, but it would usually sound unnecessary here.

Crtaju is the 3rd person plural present tense of crtati (to draw).

So:

• ja crtam = I draw
• ti crtaš = you draw
• oni/one/ona crtaju = they draw

Here:

• djeca crtaju = the children are drawing / draw

Sunce is the direct object of crtaju, so it is in the accusative singular.

The noun is:

• sunce = sun

Because sunce is a neuter noun, its nominative singular and accusative singular are the same:

• nominative: sunce
• accusative: sunce

So, just like fascikl, the case changes grammatically, but the form does not visibly change.

Because Croatian uses the instrumental case to express the tool or means used to do something.

Here the children draw the sun with a yellow felt-tip pen / marker, so Croatian uses instrumental:

• flomaster = felt-tip pen, marker
• flomasterom = with a felt-tip pen

The adjective must agree with the noun:

• žuti flomaster = a yellow marker
• žutim flomasterom = with a yellow marker

So both words are in the instrumental singular:

• žutim = instrumental singular
• flomasterom = instrumental singular

No. In Croatian, the instrumental of means or instrument often appears without s(a).

So this sentence naturally says:

• crtaju sunce žutim flomasterom

That means they draw the sun with a yellow marker.

You may also see s(a) in some contexts, but with tools and instruments, Croatian very often just uses the instrumental by itself.

Because Croatian does not have articles like English.

So Croatian simply says:

• djeca = children / the children
• naljepnice = stickers / the stickers
• fascikl = a folder / the folder
• sunce = a sun / the sun
• flomaster = a marker / the marker

English forces you to choose a or the, but Croatian usually leaves that to context.

The present tense here can work for either, depending on context.

It can mean something like:

• The children are sticking stickers on the folder and drawing the sun...
  or
• The children stick stickers on the folder and draw the sun...

Because the verbs lijepiti and crtati are imperfective, they fit very naturally with ongoing or repeated actions.

In a simple classroom-type sentence like this, English will often translate it with are sticking / are drawing.

Yes. Croatian word order is more flexible than English because case endings show grammatical roles.

This sentence uses a very natural, neutral order:

• Djeca lijepe naljepnice na fascikl i crtaju sunce žutim flomasterom.

But other orders are possible for emphasis, for example:

• Djeca na fascikl lijepe naljepnice...
• Sunce djeca crtaju žutim flomasterom...

Those alternatives change the focus or emphasis, not the basic meaning. For a learner, the given order is the most straightforward and neutral one.