Kad se stado vrati s pašnjaka, vlasnica zatvara vrata štale.

Yes. Kad and kada both mean when.

In this sentence, kad is just the shorter, very common form:

• Kad se stado vrati...
• Kada se stado vrati...

Both are correct. Kad is often a bit more natural in everyday speech.

Because the verb is vratiti se, which means to return / come back.

Without se, vratiti usually means to return something:

• Vratiti knjigu = to return a book

With se, it means the subject returns:

• Stado se vrati = the herd returns

So here se is part of the verb, not a separate word with its own meaning.

This is about Croatian clitic word order.

Words like se are clitics, and they usually come very early in the clause. After conjunctions such as kad, they often appear immediately after that conjunction:

• Kad se stado vrati... = natural and standard
• Kad stado se vrati... = not standard

So English speakers often want to place the subject first, but Croatian does not do that here.

This is an aspect question.

• vratiti se = perfective = to return, to come back, with focus on completion
• vraćati se = imperfective = to be returning / to return habitually / repeatedly

In Kad se stado vrati, the idea is when the herd has returned / once it comes back. The return is seen as completed before the next action happens.

So the sentence means something like:

• When the herd returns / once the herd has come back, the owner closes the stable door.

If you used vraća se, the meaning would feel more like the process of returning, not the completed arrival.

Because stado is a collective noun. It means a herd or a flock, and grammatically it is singular, even though it refers to many animals.

So Croatian treats it as one group:

• stado se vrati = the herd returns

This is similar to English the herd is returning, where herd is also singular.

Pašnjaka is genitive singular of pašnjak (pasture).

The preposition s/sa can mean from/off, and when it has that meaning of movement away from somewhere, it takes the genitive:

• s pašnjaka = from the pasture

So:

• na pašnjak = to the pasture
• s pašnjaka = from the pasture

Croatian often uses s/sa for movement away from open areas, surfaces, or places thought of as on something rather than in something.

A pašnjak is an open field/pasture, so s pašnjaka is the natural choice:

• s pašnjaka = from the pasture

By contrast, iz is more typical for coming out of an enclosed space:

• iz kuće = out of the house
• iz štale = out of the stable

So s pašnjaka is idiomatic Croatian.

Vlasnica means female owner.

It is the feminine form of:

• vlasnik = male owner / owner
• vlasnica = female owner

Croatian often marks gender in nouns like this, so the sentence tells you that the owner is a woman.

Yes, vrata is grammatically plural. It is a plural-only noun (pluralia tantum).

In Croatian, vrata means door or doors, but the word itself is used only in the plural form:

• vrata su zatvorena = the door is closed / the doors are closed

So in this sentence:

• zatvara vrata = closes the door / closes the doors

Very often, English singular door corresponds to Croatian vrata.

Štale is the genitive singular of štala (stable, barn).

Here it shows possession or association:

• vrata štale = the stable door or literally the door of the stable

This is a very common Croatian structure:

• prozor kuće = the house window
• vrata štale = the stable door

Because Croatian has no articles like English a and the.

So words like stado, vlasnica, vrata, and štale can mean:

• a herd / the herd
• an owner / the owner
• a door / the door

The exact meaning depends on context. In this sentence, the context makes it natural to understand them as definite.

Zatvara is the present tense of the imperfective verb zatvarati (to close).

Here it most naturally has a habitual meaning:

• Whenever the herd returns from the pasture, the owner closes the stable door.

So it describes a regular action, not just one single event.

That is why imperfective present works well here:

• vrati = completed return
• zatvara = what she regularly does then

Yes. Croatian word order is fairly flexible.

For example, you can also say:

• Vlasnica zatvara vrata štale kad se stado vrati s pašnjaka.

The meaning stays basically the same. The original version puts the when-clause first, which is very natural.

Because the sentence begins with a subordinate clause:

• Kad se stado vrati s pašnjaka, = subordinate time clause
• vlasnica zatvara vrata štale. = main clause

In Croatian, when a subordinate clause comes before the main clause, it is normally separated by a comma.