Za zabavu je izabrala masku mačke.

Because za here takes the accusative case. The base form is zabava, and its accusative singular is zabavu.

So the pattern is:

• za + accusative

In this sentence, za zabavu means something like for the party or for fun/for entertainment, depending on context.

Because je is a clitic in Croatian, and clitics usually go in the second position of the sentence or clause.

Here, the first unit is Za zabavu, so the clitic je comes right after that:

• Za zabavu je izabrala masku mačke.

This is very normal Croatian word order.

Croatian often omits subject pronouns when they are already clear from the verb form.

Here, izabrala tells you the subject is:

• singular
• feminine
• in the past tense

So she is understood without saying ona.

You could say Ona je izabrala masku mačke, but it is not necessary unless you want emphasis or contrast.

Because the verb form agrees with the subject’s gender in the past tense.

• izabrao = he chose
• izabrala = she chose
• izabralo = it chose
• izabrali / izabrale = they chose

So izabrala shows that the understood subject is feminine: she.

Because masku is the accusative singular of maska.

It is the direct object of the verb izabrala:

• maska = dictionary form, nominative
• masku = accusative singular

Since she chose a mask, the noun has to be in the accusative.

Here mačke is in the genitive singular.

The phrase maska mačke literally means mask of a cat or cat mask. In Croatian, when one noun depends on another like this, the second noun is often put in the genitive.

So:

• maska = mask
• mačke = of a cat / of the cat

This is a very common Croatian structure.

Yes, you often can.

• masku mačke = literally a mask of a cat
• mačju masku = a cat mask using an adjective derived from mačka

Both are possible, though they can sound slightly different in nuance or style. Mačju masku may sound a bit more like a standard descriptive phrase, while masku mačke is more literally the mask of a cat.

It is the perfect tense in Croatian:

• je = auxiliary verb
• izabrala = past participle

In English, this often translates simply as the simple past:

• je izabrala = she chose

So even though Croatian uses a compound tense here, the natural English translation is often just she chose.

It comes from the perfective verb izabrati.

That matters because perfective verbs usually express a completed action. Here, the choice was made; it is seen as finished.

Compare:

• izabrala je = she chose / she picked
• birala je = she was choosing / she used to choose

So izabrala fits because the sentence describes a completed selection.

Yes. Croatian word order is fairly flexible, although clitics like je still follow special placement rules.

For example, you could also say:

• Izabrala je masku mačke za zabavu.

That would still be correct, but the emphasis changes slightly. The original sentence puts Za zabavu first, which gives that part more prominence.