Na vrtuljku sjedi s prijateljicom, a poslije stavlja masku i smije se.

Because na is often used for being on a ride or attraction, and here it means on the carousel / on the merry-go-round.

• vrtuljak = carousel, merry-go-round
• after na for location, Croatian often uses the locative
• locative singular: vrtuljku

So:

• na vrtuljku = on the carousel
• but if someone is moving onto it, you would usually get the accusative: na vrtuljak

This is a very common Croatian pattern:

• na stolici = on the chair
• na autobusu can mean on the bus in some contexts, though u autobusu is also common depending on meaning

Vrtuljku is locative singular.

The noun is vrtuljak. In this sentence, it follows na in a location meaning, so Croatian uses the locative:

• nominative: vrtuljak
• locative: vrtuljku

A useful rule:

• na + locative = location
• na + accusative = motion toward / onto something

So:

• sjedi na vrtuljku = is sitting on the carousel
• sjeda na vrtuljak = is sitting down onto the carousel / getting onto it

Because the preposition s meaning with takes the instrumental case.

The base noun is:

• prijateljica = female friend

Its instrumental singular form is:

• prijateljicom

So:

• s prijateljicom = with a female friend

This is a very important pattern:

• s mamom = with mom
• s učiteljicom = with the teacher
• s prijateljem = with a male friend

So whenever you see s / sa meaning with, expect the instrumental.

Sjedi is the 3rd person singular present tense of sjediti = to sit.

So it means:

• he sits
• she sits
• is sitting

In Croatian, the present tense is often used to describe what is happening in a picture, scene, or sequence, so English might translate it as either simple present or present continuous depending on context.

Examples:

• On sjedi. = He sits / He is sitting.
• Ona sjedi na klupi. = She is sitting on the bench.

In your sentence, sjedi works like is sitting.

Because Croatian often leaves out subject pronouns when they are not needed.

The verb already shows:

• person: 3rd person
• number: singular

So sjedi, stavlja, and smije se all tell you that the subject is he/she/it. Croatian does not need to say on or ona unless there is emphasis, contrast, or a need to clarify.

So:

• Sjedi. = He/She is sitting.
• Ona sjedi. = She is sitting.
  This sounds more explicit or contrastive.

Unlike English, Croatian regularly drops pronouns.

Masku is accusative singular, because it is the direct object of the verb stavlja.

Base noun:

• maska = mask

Accusative singular:

• masku

So:

• stavlja masku = puts on a mask / is putting on a mask

For many feminine nouns ending in -a, the accusative singular changes to -u:

• knjiga → knjigu
• torba → torbu
• maska → masku

So this is a very useful pattern to remember.

Because stavlja is the imperfective present form, and it fits a scene description very naturally.

• stavljati / present stavlja = to be putting, to put repeatedly, to be in the process of placing/putting on
• staviti / present stavi = perfective verb

A key Croatian point: the present tense of perfective verbs often refers to the future, not a current ongoing action.

So:

• stavlja masku = she is putting on a mask
• stavi masku can sound more like she will put on the mask or may appear in other specific contexts

In picture descriptions and narration of ongoing actions, learners will very often see the imperfective form, like stavlja.

Because the verb here is smijati se = to laugh, and the se is part of the verb.

So:

• smije se = laughs / is laughing

This is especially important because smije by itself can also be from a different verb:

• smjeti = to be allowed to, may

So:

• Ona se smije. = She is laughing.
• Ona smije otići. = She may leave / She is allowed to leave.

That means se is not optional here. Without it, you can change the meaning completely.

A is a coordinating conjunction. It often translates as and, but it can also feel like while, but, or and then, depending on context.

In this sentence, a connects two clauses and gives a slight sense of transition or contrast:

• first: Na vrtuljku sjedi s prijateljicom
• then: a poslije stavlja masku i smije se

So here it feels like:

• and then
• and afterward
• or simply and, with a slight shift in focus

Croatian uses a a lot where English might use plain and.

Here, poslije is an adverb meaning:

• afterward
• later
• after that

So:

• a poslije stavlja masku i smije se = and afterward she puts on a mask and laughs

Important detail: poslije can also be a preposition when followed by a noun in the genitive:

• poslije škole = after school
• poslije ručka = after lunch

But in your sentence, it stands alone, so it is an adverb.

Because a is joining two separate clauses, each with its own verb:

1. Na vrtuljku sjedi s prijateljicom
2. a poslije stavlja masku i smije se

Croatian normally uses a comma before a in this kind of structure.

So the comma is not random punctuation; it reflects the clause break.

Yes, Croatian word order is fairly flexible, and the sentence can often be rearranged without changing the basic meaning.

For example, you could also say:

• S prijateljicom sjedi na vrtuljku, a poslije stavlja masku i smije se.
• Na vrtuljku s prijateljicom sjedi, a poslije stavlja masku i smije se.

But not every order sounds equally natural. The original sentence is neutral and smooth.

One thing to watch carefully is se, because it is a clitic. Clitics have preferred positions and cannot just go anywhere. So smije se is normal, while random movement of se often sounds wrong.

So yes, Croatian word order is flexible, but some small words like se still follow special rules.