Nećakinja spava s lutkom, a nećak pravi kuću od kocaka.

They are the feminine and masculine forms of the same family relationship:

• nećakinja = niece
• nećak = nephew

Croatian often marks gender directly in the noun itself, so you usually need different forms for female and male people.

Here a means something like and, while, or whereas.

So the sentence has a slight contrast or parallel idea:

• Nećakinja spava s lutkom, a nećak pravi kuću od kocaka.
• The niece is sleeping with a doll, while/and the nephew is making a house out of blocks.

If you used i, it would sound more like simple addition: and also.
With a, the sentence feels more like two separate but related scenes being compared side by side.

Because the preposition s meaning with requires the instrumental case.

The base form is:

• lutka = doll

After s, it changes to instrumental singular:

• s lutkom = with a doll

So this is a normal case change, not a different word.

Both mean with.

Usually s is the normal form, and sa is used when pronunciation is easier that way, especially before certain consonants or consonant clusters.

In this sentence:

• s lutkom is natural and easy to pronounce

Examples where sa is common:

• sa sestrom
• sa školom
• sa mnom

So here s is simply the normal choice.

Because kuću is the accusative singular of kuća.

• kuća = house, in the dictionary form
• kuću = house, as a direct object

In nećak pravi kuću, the house is the thing being made, so it must be in the accusative case.

This is very common with feminine nouns ending in -a:

• žena → ženu
• knjiga → knjigu
• kuća → kuću

Because the preposition od usually takes the genitive case and here means from or out of.

The base noun is:

• kocka = cube, block, sometimes dice depending on context

Here it becomes:

• kocaka = genitive plural

So:

• od kocaka = out of blocks / from blocks

The plural is used because the house is made from multiple blocks, not just one.

Here pravi is the present-tense form of praviti and means makes or is making.

So:

• nećak pravi kuću = the nephew makes / is making a house

In this context, English would usually say is making or is building.

Also, don’t confuse this with the adjective pravi meaning real or true.
Here it is clearly a verb because it has a direct object: kuću.

Because Croatian does not have a separate tense exactly like the English present progressive.

The present tense often covers both ideas:

• spava = sleeps / is sleeping
• pravi = makes / is making

Context tells you which English translation sounds best.

So in this sentence:

• spava is naturally understood as is sleeping
• pravi is naturally understood as is making

No, Croatian word order is fairly flexible.

The order here is neutral and natural:

• Nećakinja spava s lutkom, a nećak pravi kuću od kocaka.

But Croatian can move parts around for emphasis or style. For example, s lutkom or od kocaka could be moved if you wanted to highlight them.

Even so, the version you have is the most straightforward one for a learner.

Because Croatian has no articles.

English needs words like a, an, and the, but Croatian usually leaves that information to context.

So:

• nećakinja can mean a niece or the niece
• lutkom can mean a doll or the doll
• kuću can mean a house or the house

When translating into English, you choose the most natural article based on the situation.

A few sounds here are especially important:

• ć is a soft sound, somewhat like a very soft ch
• nj sounds like ny in canyon

So:

• nećakinja is roughly neh-chah-kinya, but with a softer ch
• kuću is roughly koo-chu, again with a soft ch

The exact Croatian sound is not identical to any single English sound, but that approximation is good for a learner at first.

Yes, literally it does mean sleeps with a doll.

In this context, though, it clearly means that the niece is sleeping with the doll beside her / cuddling it / holding it, because lutka is a doll. So there is no real ambiguity in the sentence.

For a learner, the important grammar point is that s + instrumental expresses accompaniment:

• s lutkom = with a doll