Kućna mačka živi mirno.

Kućna is an adjective meaning domestic, house, or pet in this context. It narrows the meaning of mačka.

• mačka = cat
• kućna mačka = house cat / domestic cat / pet cat

So the adjective adds extra information, just like domestic cat in English.

Because it has to agree with mačka.

In Croatian, adjectives usually match the noun in:

• gender
• number
• case

Here, mačka is:

• feminine
• singular
• nominative (the subject form)

So the adjective also takes the feminine singular nominative form:

• masculine: kućni
• feminine: kućna
• neuter: kućno

That is why we get kućna mačka.

A very common clue is the ending -a. Many Croatian nouns ending in -a are feminine.

So:

• mačka = feminine noun

That matters because adjectives and some past-tense forms will agree with it.

Be careful, though: -a is a strong clue, not an absolute rule for every noun in the language.

Croatian does not normally use articles like English a/an and the.

So:

• mačka can mean a cat, the cat, or just cat, depending on context.
• kućna mačka can mean a domestic cat, the domestic cat, or domestic cat.

Context tells you which meaning is intended.

Živi is the 3rd person singular present tense of the verb živjeti or živiti (both forms may be encountered, though živjeti is the standard form most learners are taught).

Here it means:

• he/she/it lives

Since mačka is singular, the verb is singular too:

• mačka živi = the cat lives

Some useful present forms:

• ja živim = I live
• ti živiš = you live
• on/ona/ono živi = he/she/it lives
• mi živimo = we live
• vi živite = you all / you formal live
• oni/one/ona žive = they live

Because mirno is an adverb here, not an adjective.

• miran / mirna / mirno = calm, peaceful, quiet (adjective)
• mirno = calmly, peacefully, quietly (adverb)

In the sentence, mirno describes how the cat lives, so Croatian uses the adverb:

• živi mirno = lives peacefully / lives quietly

If you said mačka je mirna, then mirna would be an adjective describing the cat itself:

• The cat is calm.

So compare:

• Kućna mačka živi mirno. = The house cat lives peacefully.
• Kućna mačka je mirna. = The house cat is calm.

It is formally identical to the neuter singular adjective form, but in this sentence it functions as an adverb.

This is very common in Croatian. Many adverbs are the same as the neuter adjective form:

• dobar / dobra / dobro → dobro = well
• brz / brza / brzo → brzo = quickly
• miran / mirna / mirno → mirno = peacefully / calmly

So although the word looks like an adjective form, its job in the sentence is adverbial.

It is in the nominative singular because it is the subject of the sentence.

The subject is the thing doing the action:

• Kućna mačka = the one that lives
• živi = lives

So the basic subject form is used:

• mačka = nominative singular

And the adjective also matches that:

• kućna mačka

Yes. Croatian word order is more flexible than English because case endings and verb forms help show grammatical relationships.

The neutral, straightforward order here is:

• Kućna mačka živi mirno.

But you could also hear:

• Mirno živi kućna mačka.
• Kućna mačka mirno živi.

These alternatives can sound more marked, poetic, or emphasize a different part of the sentence.

For a learner, the original order is the safest and most natural starting point:

• subject + verb + adverb

Kućna is pronounced roughly like KOOCH-na, but with a softer ch sound.

A few pronunciation notes:

• ku = like koo
• ć = a soft sound, somewhat like a very soft ch
• na = na

The letter ć is different from č:

• č is harder, like English ch in chocolate
• ć is softer

English does not have a perfect equivalent, so learners usually need practice hearing and repeating it.

It can be, yes, depending on context.

Kućna mačka literally means house/domestic cat, and it is a natural phrase for a cat that lives with people or for the species domestic cat.

Depending on context, Croatian speakers might also simply say:

• mačka if it is obvious they mean a pet cat

So:

• kućna mačka is correct and clear
• mačka is often enough in everyday conversation

Yes, sometimes the verb živjeti can relate to being alive, but in this sentence the meaning is clearly lives in the sense of has a way of living or exists in a certain manner.

Because of mirno:

• živi mirno = lives peacefully / lives quietly

If you want to say The cat is alive, Croatian would more naturally use:

• Mačka je živa.

So here živi is about the manner of living, not simply the state of being alive.

The dictionary forms are:

• kućni = domestic, house- (adjective; masculine singular base form)
• mačka = cat
• živjeti = to live
• mirno is usually linked to the adjective miran = calm, peaceful, and used adverbially here as peacefully / calmly

This is useful because Croatian dictionaries often list adjectives in the masculine singular form and verbs in the infinitive.