Djeca se igraju na livadi kad je vrijeme lijepo.

Djeca is the nominative plural form meaning children, and it is the subject of the sentence.

In Croatian, djeca is a special plural noun:

• nominative: djeca = children
• genitive: djece = of the children / children’s / some children in certain structures

So in this sentence, Djeca se igraju..., djeca is used because the children are the ones doing the action.

Because the subject djeca is plural.

Croatian verbs must agree with the subject in person and number.
Here:

• djeca = children = plural
• so the verb is igraju = they play / are playing

Compare:

• Dijete se igra. = The child is playing.
• Djeca se igraju. = The children are playing.

Se is a very common Croatian clitic, often used with verbs that are reflexive or middle-voice-like in meaning.

With igrati se, the full expression means:

• igrati = to play something, to perform, to play a game/instrument in some contexts
• igrati se = to play, to have fun, to engage in play

So:

• Djeca se igraju = The children are playing

You usually learn this as a set phrase:

• igrati se = to play

It does not mean that the children are literally doing something to themselves in an English reflexive sense. It is just how this verb is normally used for children playing.

Because Croatian distinguishes between:

• igrati and
• igrati se

Very roughly:

• igrati can mean things like to play a sport/game, to perform, to dance, or to play an instrument in some contexts
• igrati se means to play around / to play as children do / to have fun

So in this sentence, where children are simply playing outside, igrati se is the natural choice.

Examples:

• Djeca se igraju u parku. = The children are playing in the park.
• Igram nogomet. = I play football.
• On igra u kazalištu. = He acts/plays in the theatre.

Because after na meaning on / at in the sense of location, Croatian uses the locative case.

The noun is:

• livada = meadow

Its locative singular is:

• livadi

So:

• na livadi = on the meadow / in the meadow

This is a common pattern:

• na stolu = on the table
• na ulici = on the street
• na livadi = in/on the meadow

Both na and u can sometimes be translated as in or on, but Croatian uses them differently depending on the noun and the natural expression.

With livada (meadow), Croatian normally says:

• na livadi

This is just the idiomatic choice. English might say in a meadow, but Croatian prefers na livadi.

So this is something best learned as a natural combination:

• biti na livadi
• igrati se na livadi

Kad means when here.

So:

• kad je vrijeme lijepo = when the weather is nice

In this sentence, it gives a general time condition:

• The children play on the meadow when the weather is nice
• or The children are playing on the meadow when the weather is nice

Depending on context, it can also feel like whenever in English.

Not exactly, because kad and ako are different.

• kad = when
• ako = if

So:

• Djeca se igraju na livadi kad je vrijeme lijepo.
  = The children play on the meadow when the weather is nice.
• Djeca se igraju na livadi ako je vrijeme lijepo.
  = The children play on the meadow if the weather is nice.

Both can be possible in some contexts, but they do not mean exactly the same thing:

• kad sounds more like a regular or expected situation
• ako sounds more conditional

Both are possible.

Croatian word order is more flexible than English.
The neutral basic order is often:

• Vrijeme je lijepo. = The weather is nice.

But after kad, it is very natural to say:

• kad je vrijeme lijepo

This word order sounds smooth and normal in Croatian subordinate clauses.

So:

• kad je vrijeme lijepo
• kad vrijeme je lijepo

The first is standard and natural; the second sounds awkward.

Je is the normal short present-tense form of biti (to be) in everyday sentences.

Here:

• je = is

So:

• kad je vrijeme lijepo = when the weather is nice

The longer form jest exists, but it is usually used for emphasis, contrast, or in more formal/special contexts.

For ordinary speech:

• Vrijeme je lijepo. is normal.

It is lijepo here because it agrees with vrijeme, which is a neuter singular noun.

The adjective lijep changes form:

• masculine: lijep
• feminine: lijepa
• neuter: lijepo

Since:

• vrijeme = weather
• and vrijeme is neuter singular,

we get:

• lijepo vrijeme = nice weather
• vrijeme je lijepo = the weather is nice

Here it is functioning as an adjective/predicate adjective, not an adverb.

Why? Because it describes the noun vrijeme:

• vrijeme je lijepo = the weather is nice

Compare:

• lijepo vrijeme = nice weather
• On lijepo pjeva. = He sings beautifully.
  Here lijepo is an adverb.

So the same form can look identical, but the function is different.

It can mean either, depending on context.

Croatian present tense often covers both:

• they play
• they are playing

So:

• Djeca se igraju na livadi kad je vrijeme lijepo.

can mean:

• The children play in the meadow when the weather is nice.
• The children are playing in the meadow when the weather is nice.

In many cases, Croatian does not force the same simple vs continuous distinction that English does.

Croatian normally has no articles like English a/an/the.

So:

• djeca can mean children or the children
• livada can mean a meadow or the meadow
• vrijeme can mean weather or the weather

The exact meaning comes from context.

That is why:

• Djeca se igraju na livadi kad je vrijeme lijepo.

can naturally be understood as:

• The children are playing in the meadow when the weather is nice

even though there is no separate word for the.

Yes, Croatian allows some flexibility in word order.

For example, these are possible:

• Djeca se igraju na livadi kad je vrijeme lijepo.
• Kad je vrijeme lijepo, djeca se igraju na livadi.

The second version may sound a bit more natural if you want to emphasize the condition first:

• When the weather is nice, the children play in the meadow.

However, you cannot move small words like se completely freely.
Se is a clitic, so it usually stays near the beginning of its clause:

• Djeca se igraju... is correct
• Djeca igraju se... is much less natural and usually treated as incorrect in normal standard usage

In natural English, in the meadow is usually better.

Even though Croatian uses na, English normally says:

• The children are playing in the meadow

So this is a good example where you should not translate the preposition too literally:

• na livadi = naturally in the meadow

Croatian and English often use different prepositions for the same situation.

Vrijeme is in the nominative singular here.

It is the subject of the clause:

• kad je vrijeme lijepo = when the weather is nice

So in that clause:

• vrijeme = subject
• je = is
• lijepo = predicate adjective agreeing with vrijeme

Most naturally, it describes a general/habitual situation:

• The children play in the meadow when the weather is nice.

That is because:

• the present tense in Croatian often expresses general truths or regular actions
• kad je vrijeme lijepo sounds like a recurring condition

But depending on context, it could also describe a current scene more loosely. Still, without extra context, a habitual reading is the most natural.