Treba nam složiti kocke u kutiju prije večere.

In this sentence, treba is being used in an impersonal way.

• Treba nam složiti kocke... = We need to put the blocks in the box...
• Literally, it is closer to It is necessary for us to arrange the blocks...

Here:

• treba = it is necessary / needed
• nam = to us / for us

So treba nam + infinitive is a very common Croatian pattern for saying that something needs to be done.

You can also say moramo složiti kocke... meaning we must put the blocks away..., but that sounds a bit stronger and more like obligation. Treba nam often feels a bit softer and more practical.

Nam is the dative plural form of mi (we/us).

Relevant forms are:

• mi = we
• nas = us
• nam = to us / for us

In Treba nam složiti kocke..., nam shows who the necessity applies to:

• Treba nam = we need to / it is necessary for us

So even though English uses we as the subject, Croatian often expresses this idea with treba + dative.

Because after treba, Croatian commonly uses an infinitive to say what action is needed.

So:

• treba spavati = one needs to sleep
• treba učiti = one needs to study
• treba nam složiti kocke = we need to put the blocks in order / into the box

The infinitive is the basic to do form of the verb:

• složiti = to arrange / to put together / to tidy up / to stack, depending on context

This is about aspect, which is very important in Croatian.

• slagati = imperfective
• složiti = perfective

Very roughly:

• slagati focuses on the process, repetition, or ongoing action
• složiti focuses on completing the action

In this sentence, the idea is that the blocks should be put away before dinner, with the sense of finishing the task. That is why složiti fits well.

So:

• Treba nam slagati kocke would sound less natural here
• Treba nam složiti kocke = we need to get the blocks put away / arranged completely

It depends a little on context.

The verb složiti can mean things like:

• arrange
• put in order
• stack
• put away
• assemble

And kocke can mean:

• cubes
• blocks
• sometimes building blocks or toy blocks

So složiti kocke could mean:

• arrange the blocks
• stack the blocks
• put the blocks away neatly

Because the sentence continues with u kutiju (into the box), the most natural interpretation is something like:

• put the blocks into the box
• tidy the blocks away into the box

Kocke is the accusative plural here, because it is the direct object of složiti.

Singular and plural of kocka:

• nominative singular: kocka
• nominative plural: kocke
• accusative plural: kocke

So in this sentence:

• složiti što? = kocke
• What do we need to put away? = the blocks

For this noun, the nominative plural and accusative plural happen to look the same: kocke.

Because Croatian distinguishes between:

• movement into a place → u + accusative
• location in a place → u + locative

Here the blocks are being moved into the box, so Croatian uses accusative:

• u kutiju = into the box

Compare:

• Kocke su u kutiji. = The blocks are in the box.
  Here it is location, so kutiji is used.

So:

• u kutiju = direction / destination
• u kutiji = position / location

Kutiju is the accusative singular of kutija (box).

Forms:

• nominative: kutija
• accusative: kutiju

It is accusative because after u meaning into, Croatian uses the accusative.

So:

• u kutiju = into the box

The preposition prije (before) takes the genitive.

So:

• večera = dinner
• večere = genitive singular

That is why you get:

• prije večere = before dinner

This is something you mostly just learn with the preposition:

• prije škole = before school
• prije ručka = before lunch
• prije večere = before dinner

Croatian often leaves the subject unstated when it is clear from context.

In this sentence, English uses we, but Croatian expresses the idea differently:

• Treba nam... = We need... / It is necessary for us...

So the idea of we/us is already contained in nam. There is no need to add mi unless you want emphasis.

For example:

• Treba nam složiti kocke. = We need to put the blocks away.
• Mi trebamo složiti kocke. = We need to put the blocks away.
  This is possible, but it is a different structure and may sound more direct or emphatic.

Yes. Croatian word order is fairly flexible, although some orders sound more natural than others depending on emphasis.

The original sentence:

• Treba nam složiti kocke u kutiju prije večere.

Possible variations include:

• Kocke treba nam složiti u kutiju prije večere.
• Prije večere treba nam složiti kocke u kutiju.
• Treba nam prije večere složiti kocke u kutiju.

These all keep roughly the same basic meaning, but the emphasis changes slightly.

The original version is natural and neutral.

Both can mean something like We need to put the blocks in the box before dinner, but the tone is a little different.

• Treba nam složiti... = we need to, it is necessary for us to
• Moramo složiti... = we must, we have to

So:

• treba nam often sounds a bit softer, more practical, or more like a requirement of the situation
• moramo sounds more direct and stronger

In many everyday situations, both are possible, but they are not always identical in tone.

Sometimes kocka can refer to a cube-shaped object more generally, and in some contexts related words can connect to dice, but in this sentence kocke is most naturally understood as blocks or toy blocks.

That is especially true because of:

• složiti kocke
• u kutiju
• prije večere

This sounds like tidying up children's blocks before dinner, not putting dice into a box.

A more literal translation would be:

• It is necessary for us to arrange the blocks into the box before dinner.

That sounds unnatural in normal English, so the natural translation is more like:

• We need to put the blocks in the box before dinner.
• We need to tidy the blocks away into the box before dinner.

Seeing the literal structure can help explain the grammar:

• Treba = it is necessary
• nam = for us / to us
• složiti = to arrange / put away completely
• kocke = the blocks
• u kutiju = into the box
• prije večere = before dinner