Poslije ručka nećakinja pravi mačku od plastelina.

Because poslije takes the genitive case.

• dictionary form: ručak = lunch
• genitive singular: ručka

So:

• poslije ručka = after lunch

This is very common in Croatian: certain prepositions require a specific case, and poslije is one of the prepositions that uses the genitive.

Poslije means after. In this sentence, it introduces a time expression:

• Poslije ručka = after lunch

Yes, you can also say nakon ručka. Both are natural and common.

A rough comparison:

• poslije = after
• nakon = after

In many everyday contexts, they are interchangeable.

Because nećakinja is in the nominative form, which is the usual case for the subject.

• nećakinja = niece

In the sentence, she is the person doing the action:

• nećakinja pravi... = the niece makes / is making...

Croatian often relies on case endings, not just word order, to show who is doing what.

Because praviti means to make, create, form, while raditi usually means to work, do, be working.

Here the niece is making something:

• pravi mačku = she is making a cat

So praviti is the natural verb.

Compare:

• Pravi tortu. = She is making a cake.
• Radi domaću zadaću. = She is doing homework.
• Radi u školi. = She works at a school.

Because it is the direct object, so it appears in the accusative case.

• nominative: mačka = cat
• accusative singular: mačku

This is a very common pattern for many feminine nouns ending in -a:

• žena → ženu
• kuća → kuću
• mačka → mačku

So:

• nećakinja pravi mačku = the niece is making a cat

Because od is the usual preposition for saying what something is made of.

• od plastelina = out of plasticine / made of plasticine

Also, od requires the genitive case, so:

• dictionary form: plastelin
• genitive singular: plastelina

That is why it is od plastelina, not od plastelin.

Yes, pravi is present tense.

Croatian present tense can often translate into English in more than one way depending on context:

• makes
• is making

So this sentence could mean either:

• After lunch, the niece makes a cat out of plasticine.
• After lunch, the niece is making a cat out of plasticine.

In real usage, context tells you whether it is a habitual action or something happening right now / in a specific situation.

Yes. Croatian word order is fairly flexible because cases show the grammatical roles.

The neutral order here is:

• Poslije ručka nećakinja pravi mačku od plastelina.

But other orders are also possible, depending on emphasis, for example:

• Nećakinja poslije ručka pravi mačku od plastelina.
• Mačku od plastelina nećakinja pravi poslije ručka.

These versions do not all sound equally neutral, but they are grammatical. Croatian uses word order more for focus and emphasis than English does.

Because of the case ending.

• nećakinja is nominative, so it is the subject
• mačku is accusative, so it is the direct object

Even if the word order changed, the endings would still help show the roles.

That is one of the most important things to get used to in Croatian: endings carry a lot of grammatical information.

Yes. Nećakinja means niece, so it is specifically female.

The male counterpart is:

• nećak = nephew

So:

• nećakinja = niece
• nećak = nephew

The main letters English speakers often notice here are:

• č in ručka and mačku
• ć in nećakinja
• j in pravi, nećakinja, poslije

A simple guide:

• č sounds roughly like the ch in church, but firmer
• ć is also a ch-like sound, but softer
• j is like English y in yes

So:

• poslije ≈ POS-lyeh
• ručka ≈ ROOCH-ka
• nećakinja ≈ neh-CHA-kin-ya (very approximate)
• pravi ≈ PRA-vee
• mačku ≈ MACH-koo
• plastelina ≈ plas-teh-LEE-na

These are only rough approximations, but they can help at first.

Yes. If the material is clear from context, you can simply say:

• Poslije ručka nećakinja pravi mačku.

That means After lunch, the niece is making a cat.

Adding od plastelina just gives extra information about the material:

• ...pravi mačku od plastelina = ...is making a cat out of plasticine

Because Croatian has no articles.

English distinguishes:

• a cat
• the cat

Croatian usually does not mark that difference with a separate word. Instead, the meaning comes from context.

So mačku could correspond to:

• a cat
• the cat

depending on the situation.