Poslije škole dječak i djevojčica sjede na livadi.

Because poslije takes the genitive case, and the genitive singular of škola is škole.

So:

• škola = nominative singular
• škole = genitive singular

This is very common in Croatian: many prepositions require a specific case, and poslije is one of the prepositions that requires the genitive.

It is genitive singular.

The noun is škola and after poslije it changes to škole:

• škola → nominative
• škole → genitive singular

So poslije škole is a standard preposition + case combination.

Yes. Nakon škole is also correct and means the same thing.

Both poslije škole and nakon škole are natural Croatian. In many contexts they are interchangeable. A learner should recognize both.

Because the subject is dječak i djevojčica — two people joined by i (and).

Even though each noun is singular on its own:

• dječak = singular
• djevojčica = singular

together they form a plural subject, so the verb must also be plural:

• sjede = they sit / are sitting

Because sjede is simply the correct 3rd person plural present form of sjediti.

Present tense of sjediti:

• ja sjedim
• ti sjediš
• on/ona sjedi
• mi sjedimo
• vi sjedite
• oni/one sjede

So in this sentence, since the subject is they, the correct form is sjede.

This is an important Croatian distinction.

• sjediti = to be sitting, to sit as a state
• sjesti = to sit down as an action

So:

• Dječak i djevojčica sjede = they are sitting
• Dječak i djevojčica sjednu / sjest će = they sit down / will sit down

English often uses sit for both ideas, but Croatian usually separates:

• state → sjediti
• movement into that state → sjesti

Because after na, when you are talking about location, Croatian uses the locative case.

Here the sentence describes where they are sitting, so livada changes to livadi:

• livada = nominative singular
• livadi = locative singular

So:

• na livadi = on/in the meadow, at the meadow area

Na livadu uses the accusative case and usually shows movement toward a place.

Compare:

• sjede na livadi = they are sitting on the meadow / in the meadow → location
• idu na livadu = they are going to the meadow → movement

This is a very useful pattern:

• na + locative = location
• na + accusative = movement/destination

No, Croatian word order is fairly flexible.

Poslije škole dječak i djevojčica sjede na livadi is natural and neutral, but other orders are possible, depending on emphasis.

For example:

• Dječak i djevojčica sjede na livadi poslije škole.
• Na livadi sjede dječak i djevojčica poslije škole.

These do not usually change the basic meaning, but they can change what sounds most emphasized or most natural in context.

Because Croatian does not have articles like English a and the.

So:

• dječak can mean a boy or the boy
• djevojčica can mean a girl or the girl
• livada can mean a meadow or the meadow

The exact interpretation depends on context.

Here it means after school in the time sense.

That is because:

• poslije means after in a temporal sense here
• škole is in the genitive after that preposition

If you want to say behind the school in a physical, spatial sense, Croatian would usually use:

• iza škole

So:

• poslije škole = after school
• iza škole = behind the school

In Croatian, the present tense can cover both ideas, depending on context.

So sjede can correspond to:

• sit
• are sitting

In this sentence, English often prefers are sitting, because it describes what they are doing at that moment. But grammatically Croatian just uses the normal present tense form sjede.