U kokošinjcu je bilo toplo, pa su kokoši mirno sjedile unutra.

Because u meaning in takes the locative case when it shows location.

• kokošinjac = chicken coop / henhouse
• locative singular: kokošinjcu

So:

• u kokošinjcu = in the chicken coop

A useful contrast:

• u + accusative = movement into
• u + locative = being in

For example:

• Ušla je u kokošinjac. = She went into the coop.
• Bila je u kokošinjcu. = She was in the coop.

Kokošinjac is a noun meaning chicken coop, henhouse, or place where chickens are kept.

Its basic form is:

• kokošinjac = singular, nominative

In this sentence it appears as:

• u kokošinjcu = locative singular

A learner might also notice the related word:

• kokoš = hen / chicken

So kokošinjac is clearly built around the idea of a place for kokoši.

Because bilo toplo is an impersonal expression meaning it was warm.

Croatian often uses bilo je + adjective/adverb-like predicate to describe general conditions:

• Bilo je hladno. = It was cold.
• Bilo je tiho. = It was quiet.
• Bilo je toplo. = It was warm.

Here, the sentence is not saying that the coop itself was some grammatically masculine thing that must agree with topao. Instead, it is describing the overall condition inside it: it was warm there.

So:

• U kokošinjcu je bilo toplo = It was warm in the coop

This is a very common Croatian pattern.

Because in this kind of impersonal construction, Croatian uses the neuter singular form:

• topao = masculine singular
• topla = feminine singular
• toplo = neuter singular

In bilo je toplo, toplo works as the standard form used for general states or conditions, similar to English it was warm.

You will often see:

• Vani je hladno. = It is cold outside.
• Ovdje je ugodno. = It is pleasant here.
• U sobi je toplo. = It is warm in the room.

So toplo is exactly what you expect here.

Here pa means something like:

• so
• and so
• therefore
• sometimes simply and then, depending on context

In this sentence, it links the two ideas with a sense of consequence:

• It was warm in the coop, so the hens sat quietly inside.

So pa is not just a plain and. It suggests that the second part follows naturally from the first.

Compare:

• i = and
• ali = but
• pa = so / and so / then

Because su is a clitic in Croatian, and clitics usually go in the second position of their clause.

In the clause:

• pa su kokoši mirno sjedile unutra

the conjunction pa comes first, and the clitic su comes right after it.

This is very normal Croatian word order.

So:

• pa su kokoši... = natural
• pa kokoši su... = not the normal choice here

Croatian clitics include forms like:

• sam, si, je, smo, ste, su
• se
• short object pronouns like ga, je, ih, etc.

This is one of the most important word-order patterns for learners.

Because Croatian standard past tense is usually built with:

• the present tense of biti (sam, si, je, smo, ste, su)
• plus the l-participle

Here:

• su = they are in the auxiliary function
• sjedile = participle

Together:

• su sjedile = they were sitting / they sat

This is the normal perfect-like past construction in Croatian.

Examples:

• Ona je došla. = She came.
• Mi smo čekali. = We waited.
• Kokoši su sjedile. = The hens sat / were sitting.

Because sjedile agrees with kokoši, which is feminine plural.

The subject is:

• kokoši = hens / chickens

The participle must match it in gender and number:

• masculine singular: sjedio
• feminine singular: sjedila
• neuter singular: sjedilo
• masculine plural: sjedili
• feminine plural: sjedile
• neuter plural: sjedila in some agreement contexts, though learners mostly meet the other forms first

So:

• kokoši su sjedile = the hens were sitting

That -e ending is there because the subject is feminine plural.

Because sjediti and sjesti are different in aspect and meaning.

• sjediti = to sit, to be sitting → imperfective
• sjesti = to sit down → perfective

In this sentence, the hens are already inside and sitting there calmly. The focus is on the ongoing state, not the moment of sitting down.

So:

• kokoši su mirno sjedile unutra = the hens were sitting quietly inside

If you used sjele, it would mean something more like:

• the hens sat down inside

That would shift the meaning to the action of taking a seat, not the state of being seated.

Mirno is an adverb meaning:

• calmly
• quietly
• peacefully

It comes from the adjective miran = calm, peaceful.

Here it modifies sjedile:

• mirno sjedile = sat quietly / sat calmly

So it describes how the hens were sitting.

Compare:

• miran pas = a calm dog
• pas mirno leži = the dog is lying calmly

Unutra is an adverb meaning:

• inside
• indoors
• in there

So the sentence uses:

• u kokošinjcu = in the coop
• unutra = inside

This may feel slightly repetitive in English, but it is perfectly natural in Croatian. It reinforces the idea that the hens stayed inside, not outside.

You could think of it as:

• In the coop it was warm, so the hens sat quietly inside.

Croatian often uses adverbs like:

• unutra = inside
• vani / vani = outside
• gore = up / upstairs
• dolje = down / downstairs

Croatian word order is fairly flexible, but different orders sound more or less natural depending on emphasis.

The given sentence is very natural:

• U kokošinjcu je bilo toplo, pa su kokoši mirno sjedile unutra.

Possible variations include:

• Bilo je toplo u kokošinjcu, pa su kokoši mirno sjedile unutra.
• U kokošinjcu je bilo toplo, pa su unutra kokoši mirno sjedile.
  This is possible, but less neutral and more marked.

The original version is good because:

1. it starts with the setting: U kokošinjcu
2. then gives the condition: je bilo toplo
3. then the result: pa su kokoši...

So yes, the order can change, but the original is very natural and well-structured.

Most naturally, kokoši means hens here.

Singular:

• kokoš = hen

Plural:

• kokoši = hens

In some contexts, English might translate it more loosely as chickens, especially if the exact sex is not important. But strictly speaking, kokoš usually refers to a hen, not just any chicken.

So in this sentence, hens is probably the most precise translation.

Because Croatian usually does not need subject pronouns when the verb form already shows the person and number.

Here:

• su already tells you it is they
• sjedile also agrees with a plural feminine subject

And the subject noun is explicitly present anyway:

• kokoši

So adding one would be unnecessary unless you wanted special emphasis.

This is normal in Croatian:

• Došli su. = They came.
• Radimo. = We are working.
• Ona je stigla. = She arrived.
  Even here, ona is used only if needed for clarity or emphasis.

It mainly describes an ongoing situation in the past.

• je bilo toplo = it was warm
• su sjedile = they were sitting

The imperfective verb sjediti especially suggests a state or ongoing action rather than a single completed event.

So the overall picture is:

• the coop was warm
• as a result, the hens remained sitting quietly inside

It is more like a scene being described than a sudden action sequence.