Dječak čvrsto drži crveni balon.

Dječak is in the nominative singular because it is the subject of the sentence: the boy is the one doing the action.

The basic dictionary form is dječak = boy.

In this sentence:

• dječak = subject
• drži = verb
• balon = object

So the structure is basically subject + adverb + verb + object.

Držati is the infinitive, meaning to hold.

Drži is the 3rd person singular present tense form, meaning he/she/it holds.

It matches dječak, which is he:

• ja držim = I hold
• ti držiš = you hold
• on/ona drži = he/she holds

So Dječak drži... literally means The boy holds...

Čvrsto is an adverb. It describes how the boy is holding the balloon.

Here it means something like:

• firmly
• tightly

It comes from the adjective čvrst, which means firm, solid, or tight depending on context.

So:

• čvrst = firm, solid
• čvrsto = firmly, tightly

Because the adjective has to agree with the noun in gender, number, and case.

Here:

• balon is masculine
• singular
• accusative singular as the direct object

So the adjective also takes the matching form:

• crveni balon = red balloon

A useful extra point: in modern Croatian, masculine singular adjectives before a noun often appear in the long form, so crveni is the normal form here.

Compare:

• crveni balon = red balloon
• Balon je crven. = The balloon is red.

Because balon is a masculine inanimate noun.

In Croatian, masculine inanimate nouns usually have the same form in the nominative singular and accusative singular.

So:

• nominative: balon
• accusative: balon

That is why the object still looks like balon.

Compare that with a masculine animate noun:

• dječak = boy
• Vidim dječaka. = I see the boy

Here the accusative changes to dječaka because it is animate.

Croatian does not have articles like English a and the.

So Dječak drži crveni balon could mean:

• A boy is holding a red balloon
• The boy is holding the red balloon

The exact meaning depends on the context.

English forces you to choose an article, but Croatian usually leaves that information to the situation or to other words in the sentence.

Croatian word order is fairly flexible because case endings show what each word is doing.

The version you have:

• Dječak čvrsto drži crveni balon.

is a very neutral and natural order.

But other orders are possible for emphasis, such as:

• Dječak drži crveni balon čvrsto.
• Crveni balon dječak čvrsto drži.

These do not usually change the basic meaning, but they can change the focus or style.

For learners, the given order is a good standard model.

A rough English-friendly guide:

• dječak ≈ DYE-chak
• čvrsto ≈ CHVR-sto
• drži ≈ DR-zhee

Some key sounds:

• č sounds like ch in church
• ž sounds like the s in measure
• r in Croatian is usually a tapped or rolled r

Also, Croatian spelling is quite regular, so once you learn the sound of each letter, pronunciation becomes much easier.

It can do either, depending on context.

Because držati is an imperfective verb, the present tense can describe:

• an action happening right now
• a habitual or repeated action
• a general state

In this sentence, the most natural reading is probably:

• The boy is holding the red balloon tightly.

But in the right context, it could also mean something more general, like:

• The boy holds the red balloon tightly.

It can mean several related things, depending on context.

Common meanings include:

• to hold
• to keep
• to support
• to maintain

For example:

• drži knjigu = holds a book
• drži novac u ladici = keeps money in a drawer
• drži vrata otvorena = keeps the door open

In your sentence, the meaning is clearly hold.